These are representative sample records from Science.gov related to your search topic.
For comprehensive and current results, perform a real-time search at Science.gov.
1

Increased induction in FeCo-based nanocomposite materials with reduced early transition metal growth inhibitors  

E-print Network

Increased induction in FeCo-based nanocomposite materials with reduced early transition metal-temperature nanocomposite alloys with reduced glass formers. The amounts of the magnetic transition metals and early transition metal growth inhibitors were systematically varied to determine trade-offs between higher

McHenry, Michael E.

2

Density functional plus dynamical mean-field theory of the metal-insulator transition in early transition-metal oxides  

NASA Astrophysics Data System (ADS)

The combination of density functional theory and single-site dynamical mean-field theory, using both Hartree and full continuous-time quantum Monte Carlo impurity solvers, is used to study the metal-insulator phase diagram of perovskite transition-metal oxides of the form ABO3 with a rare-earth ion A =Sr, La, Y and transition metal B =Ti, V, Cr. The correlated subspace is constructed from atomiclike d orbitals defined using maximally localized Wannier functions derived from the full p-d manifold; for comparison, results obtained using a projector method are also given. Paramagnetic DFT + DMFT computations using full charge self-consistency along with the standard "fully localized limit" (FLL) double counting are shown to incorrectly predict that LaTiO3, YTiO3, LaVO3, and SrMnO3 are metals. A more general examination of the dependence of physical properties on the mean p-d energy splitting, the occupancy of the correlated d states, the double-counting correction, and the lattice structure demonstrates the importance of charge-transfer physics even in the early transition-metal oxides and elucidates the factors underlying the failure of the standard approximations. If the double counting is chosen to produce a p-d splitting consistent with experimental spectra, single-site dynamical mean-field theory provides a reasonable account of the materials properties. The relation of the results to those obtained from "d-only" models in which the correlation problem is based on the frontier orbital p-d antibonding bands is determined. It is found that if an effective interaction U is properly chosen the d-only model provides a good account of the physics of the d1 and d2 materials.

Dang, Hung T.; Ai, Xinyuan; Millis, Andrew J.; Marianetti, Chris A.

2014-09-01

3

The standard enthalpies of formation of the compounds of early transition metals with late transition metals and with noble metals as determined by Kleppa and co-workers at the University of Chicago — A review  

Microsoft Academic Search

Since the early 1980s, experimental studies of the standard enthalpies of formation of the binary intermetallic compounds of early transition metals with late transition metals and with noble metals have been a major long-term research project in this laboratory. Tabulated in this review are 290 enthalpy of formation values for 273 such compounds, all determined in this laboratory during the

Qiti Guo; Ole Jakob Kleppa

2001-01-01

4

Hydrogen-atom abstraction from methane by stoichiometric early transition metal oxide cluster cations.  

PubMed

Stoichiometric early transition metal oxide cations (TiO(2))(1-5)(+), (ZrO(2))(1-4)(+), (HfO(2))(1-2)(+), (V(2)O(5))(1-5)(+), (Nb(2)O(5))(1-3)(+), (Ta(2)O(5))(1-2)(+), (MoO(3))(1-2)(+), (WO(3))(1-3)(+), and Re(2)O(7)(+) are able to activate the C-H bond of methane under near room temperature conditions. PMID:20177633

Zhao, Yan-Xia; Wu, Xiao-Nan; Wang, Zhe-Chen; He, Sheng-Gui; Ding, Xun-Lei

2010-03-14

5

Transition metals  

PubMed Central

Transition metals such as Iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. In this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested. PMID:23333964

Rodrigo-Moreno, Ana; Poschenrieder, Charlotte; Shabala, Sergey

2013-01-01

6

First Principles Calculation of Elastic Properties of Early-Late Transition Metal Alloys  

NASA Astrophysics Data System (ADS)

Amorphous metals are of practical interest in applications requiring high strength materials. We choose to examine the elastic properties of crystalline phases to understand the elastic properties of amorphous solids. In this talk, we discuss our work using first principles methods to calculate elastic properties for crystalline alloys in various chemical families containing transition metals, specifically early (Ta,W) and late (Fe,Co,Rh,Ni,Cu,Zn) due to their good glass forming ability, as well as select borides. Certain Laves phases, which are known to have local chemical ordering similar to amorphous solids, are focused on. We analyze trends in the elastic properties of chemical families based on computed enthalpies of formation, elastic properties of pure elemental phases, and electronic and structural information. In particular, we use effective medium theories and enthalpies of formation to predict trends in bulk moduli. This information can be used to predict future candidate systems for high-strength amorphous metals.

Huhn, William; Widom, Michael

2013-03-01

7

Early Transition Metal Oxides as Catalysts: Crossing Scales from Clusters to Single Crystals to Functioning Materials  

SciTech Connect

The overall goal of this program is to investigate the electronic structure and chemical bonding of early transition metal oxide clusters and use them as well-defined molecular models to obtain insight into properties and mechanisms of oxide catalysts, as well as to provide accurate spectroscopic and molecular information to verify theoretical methods used to predict materials properties. A laser vaporization cluster source is used to produce metal oxide clusters with different sizes, structures, and compositions. Well-defined inorganic polyoxometalate clusters in solution are transported in the gas phase using electrospray. Two state-of-the-art photoelectron spectroscopy apparatuses are used to interrogate the oxide clusters and polyoxometalate anions in the gas phase to obtain spectroscopic and electronic structure information. The experimental effort is assisted by theoretical calculations to understanding the structures, chemical bonding, and catalytical properties of the transition metal oxide clusters. The research approach combines novel and flexible experimental techniques and advanced theoretical/computational methodologies and seeks molecular-level information to aiding the design of new catalysts, as well as mechanistic understanding. We have focused on the investigation of tungsten oxide clusters containing three W atoms: W{sub 3}O{sub x}{sup -} (x = 7-11). A number of interesting findings have been made. We observed that the oxygen-poor W{sub 3}O8 cluster contains a localized W{sup 4+} center, which can be used as a molecular model for O-deficient defect sites. A chemisorption energy was obtained through density functional calculations for W{sub 3}O8 + O{sub 2} {yields} W{sub 3}O{sub 10} as -78 kcal/mol. We further found that the neutral stoichiometric W{sub 2}O{sub 6} and W{sub 3}O{sub 9} clusters do not react with O{sub 2} and they only form physi-sorbed complexes, W{sub 2}O{sub 6}(O{sub 2}) and W{sub 3}O{sub 9}(O{sub 2}). However, the negatively charged W{sub 2}O{sub 6}{sup -} and W{sub 3}O{sub 9}{sup -} clusters are found to form chemisorbed complexes due to the presence of the extra electron. Thus, the W{sub 2}O{sub 6}{sup -} and W{sub 3}O{sub 9}{sup -} negative clusters can be viewed as models for O{sub 2} interaction with a reduced W site (W{sup 5+}) on the oxide surface. These studies also led to the surprising observation of the first d-orbital aromatic clusters in W{sub 3}O{sub 9}{sup 2-} and Mo{sub 3}O{sub 9}{sup 2-}, which each contains a completely delocalized three-center two-electron bond made entirely made of the metal d orbitals. This last result was highlighted in both Chem & Eng. News and Nature. We further studied a series of small metalate anions using electrospray, including the hydroxo and methoxo oxometalate MO{sub 3}(OH){sup -} and MO{sub 3}(OCH{sub 3}){sup -}, and the dimetalates: M{sub 2}O{sub 7}{sup 2-}, MM{prime}O{sub 7}{sup 2-}, and M{sub 2}O{sub 7}{sup -} (M, M{prime} = Cr, Mo, and W).

Lai-Sheng Wang

2009-07-07

8

FINAL TECHNICAL REPORT Synthetic, Structural and Mechanistic Investigations of Olefin Polymerization Catalyzed by Early Transition Metal Compounds  

SciTech Connect

The goal of this project is to develop new catalysts and provide understanding of ligand effects on catalyst composition in order to guide development of superior catalyst systems for polymerization of olefins. Our group is designing and synthesizing new “LX2”,“pincer” type ligands and complexing early transition metals to afford precatalysts. In a collaboration with Hans Brintzinger from the University of Konstanz, we are also examining the structures of the components of catalyst systems obtained from reaction of zirconocene dichlorides with aluminum alkyls and aluminum hydrides. Such systems are currently used commercially to produce polyolefins, but the nature of the active and dormant species as well as the mechanisms of their interconversions are not understood. New information on catalyst design and performance may lead to new types of polymers and/or new chemical transformations between hydrocarbons and transition metal centers, ultimately contributing to the development of catalytic reactions for the production of fuels, commodity and polymeric materials.

Bercaw, John E. [California Institute of Technology

2014-05-23

9

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group  

NASA Astrophysics Data System (ADS)

Aims: We focus on the resolved stellar populations of one early-type and four transition-type dwarf galaxies in the Sculptor group, with the aim to examine the potential presence of population gradients and place constraints on their mean metallicities. Methods: We use deep Hubble Space Telescope images to construct color-magnitude diagrams, from which we select stellar populations that trace different evolutionary phases in order to constrain their range of ages and metallicities, as well as to examine their spatial distribution. In addition, we use the resolved stars in the red giant branch in order to derive photometric metallicities. Results: All studied dwarfs contain intermediate-age stars with ages of ~1 Gyr and older as traced by the luminous asymptotic giant branch and red clump stars, while the transition-type dwarfs contain also stars younger than ~1 Gyr as traced by a young main sequence and vertical red clump stars. Moreover, the spatial distribution of the stars that trace different evolutionary phases shows a population gradient in all transition-type dwarfs. The derived error-weighted mean metallicities, assuming purely old stellar populations, range from -1.5 dex for ESO294-G010 to -1.9 dex for Scl-dE1, and should be considered as lower limits to their true metallicities. Assuming intermediate-age stellar populations to dominate the dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex higher than the metallicities derived assuming purely old populations. We discuss how photometric metallicity gradients are affected by the age-metallicity degeneracy, which prevents strong conclusions regarding their actual presence. Finally, the transition-type dwarfs lie beyond the virial radius of their closest bright galaxy, as also observed for the Local Group transition-type dwarfs. Scl-dE1 is the only dwarf spheroidal in our sample and is an outlier in a potential morphology-distance relation, similar as the two isolated dwarf spheroidals of the Local Group, Tucana, and Cetus. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Institute. STScI is operated by the association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.Figures 2, 5, 6 and Appendices A-C are available in electronic form at http://www.aanda.orgPhotometry tables are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A7

Lianou, S.; Grebel, E. K.; Da Costa, G. S.; Rejkuba, M.; Jerjen, H.; Koch, A.

2013-02-01

10

Bonding in diborane-metal complexes: a quantum-chemical and experimental study of complexes featuring early and late transition metals.  

PubMed

The coordination chemistry of the doubly base-stabilised diborane(4), [HB(hpp)]2 (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2-a]pyrimidinate), was extended by the synthesis of new late transition-metal complexes containing Cu(I) and Rh(I) fragments. A detailed experimental study was conducted and quantum-chemical calculations on the metal-ligand bonding interactions for [HB(hpp)]2 complexes of Group 6, 9, 11 and 12 metals revealed the dominant B-H-M interactions in the case of early transition-metal fragments, whereas the B-B-M bonding prevails in the case of the late d-block compounds. These findings support the experimental results as reflected by the IR and NMR spectroscopic parameters of the investigated compounds. DFT calculations on [MeB(hpp)]2 and model reactions between [B2H4?2NMe3] and [Rh(?-Cl)(C2H4)2] showed that the bicyclic guanidinate allows in principle for an oxidative addition of the B-B bond. However, the formation of ?-complexes is thermodynamically favoured. The results point to the selective B-H or B-B bond-activation of diborane compounds by complexation, depending on the chosen transition-metal fragment. PMID:23576458

Wagner, Arne; Kaifer, Elisabeth; Himmel, Hans-Jörg

2013-06-01

11

Bacterial Transition Metal Homeostasis  

Microsoft Academic Search

Since details on metal cation transport proteins and on the allocation mechanisms for transition metals\\u000a are provided elsewhere in this book, I will present aspects of transition metal homeostasis in a hopefully\\u000a novel overview. We will start with a microbial look at the transition metal Periodic Table, cation speciation,\\u000a and availability in the environment. This information provides rules that might

Dietrich H. Nies

12

Ligand and initiator effect in late and early transition metal catalyzed living radical polymerizations  

NASA Astrophysics Data System (ADS)

This thesis was concerned with the development of novel catalysts and initiators for living radical polymerization (LRP). The effect of metal, ligand and initiator was studied in the LRP of styrene using transition metal porphyrins as well as a series of 18 Ti complexes with O, N as well as substituted cyclopentadienyl ligands in conjunction with epoxide and halide initiators. The nature of the metal center was first explored in a series of tetraphenyl porphyrins containing Cu, Ni, Pb, Pd, Pt, Zn, and Ag. Styrene polymerization in the presence of these catalysts showed a linear dependence of molecular weight (Mn) on conversion but also gave broad polydisperisties (PDI). This behavior was attributed to a copolymerization of the porphyrin with styrene. Subsequently, the Cp2TiCl-catalyzed radical ring opening (RRO) of epoxides was introduced as a novel initiating reaction for LRP mediated by the unprecedented reversible end-capping of the propagating chain by Ti(III). These results were supported by a combination of NMR, GPC and reinitiation experiments and generated polymers with very narrow PDI (<1.2) and functional OH chain ends. Ti(III) was also shown to be compatible with activated and unactivated halides yielding the first halide initiated LRP which does not follow an atom transfer polymerization mechanism. Further studies of the stereoelectronic ligand effects in RRO and LRP ranked these catalysts as: L = metallocene (eta5-RCp: R = H ˜ Et ˜ iPr ˜ tBu > Ind >> Cp*) >> O-ligands (alkoxides > bisketonates) >> N ligands (hydrotris(pyrazol-1-ylborato) > phthalocyanine), while X = Cl > Br >> F ˜ Me ˜ CO and LnTiCl m > Ln-1TiClm+1. Additional experimentation examined the effect of reaction conditions and optimum results were obtained with [St]/[epoxide]/[Ti]/[Zn] = 50/1/4/8 at 60 °C--90 °C in dioxane using a 1,4-butandiol diglycidyl ether. Finally, the first example of a dual concurrent polymerization of styrene and epsilon-caprolactone was demonstrated using a single catalyst/initiator system (Ti/epoxide) and new criteria for quantifying the livingness of the process were outlined.

Moran, Isaac Webster

13

Metal-insulator transitions  

Microsoft Academic Search

Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by

Masatoshi Imada; Atsushi Fujimori; Yoshinori Tokura

1998-01-01

14

Oligocyclopentadienyl transition metal complexes  

SciTech Connect

Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.

de Azevedo, Cristina G.; Vollhardt, K. Peter C.

2002-01-18

15

Mott Metal Insulator Transitions  

E-print Network

Physics Letters 95, no. 4 (2009) 14 #12;Mott Insulators: Actuators Bimorph Actuators Thermal Sensing LiuMott Metal Insulator Transitions Satej Soman, Robert Tang-Kong March 21, Physics 141A 1 #12;Definitions: What is a Mott Insulator? Metal that stops conducting at low temperature or high pressure

Budker, Dmitry

16

Fine tuning of metallaborane geometries: chemistry of metallaboranes of early transition metals derived from metal halides and monoborane reagents.  

PubMed

Reaction of [Cp(n)MCl(4-x)] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl(4)] (Cp=eta(5)-C(5)H(5), Cp*=eta(5)-C(5)Me(5)), with [LiBH(4).thf] at -70 degrees C followed by thermolysis at 85 degrees C in the presence of [BH(3).thf] yielded the hydrogen-rich metallaboranes [(CpM)(2)(B(2)H(6))(2)] (1: M=V; 2: M=Nb) and [(Cp*Ta)(2)(B(2)H(6))(2)] (3) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal-metal bond bridged by two hexahydroborate (B(2)H(6)) groups forming a symmetrical complex. Addition of [BH(3).thf] to 3 results in formation of a metallaborane [(Cp*Ta)(2)B(4)H(8)(mu-BH(4))] (4) containing a tetrahydroborate ligand, [BH(4)](-), bound exo to the bicapped tetrahedral cage [(Cp*Ta)(2)B(4)H(8)] by two Ta-H-B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B-H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, (1)H, (11)B, and (13)C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1-4. PMID:19894230

Bose, Shubhankar Kumar; Geetharani, K; Ramkumar, V; Mobin, Shaikh M; Ghosh, Sundargopal

2009-12-14

17

Transition Metal Switchable Mirror  

ScienceCinema

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

None

2010-01-08

18

Transition Metal Switchable Mirror  

SciTech Connect

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm

None

2009-01-01

19

Transition Metal Switchable Mirror  

SciTech Connect

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

2009-08-21

20

Calixarene supported transition metal clusters   

E-print Network

This thesis describes a series of calix[n]arene polynuclear transition metal and lanthanide complexes. Calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition metal ...

Taylor, Stephanie Merac

2013-06-29

21

Metal to semiconductor transition in metallic transition metal dichalcogenides  

SciTech Connect

We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX{sub 2} (M?=?Nb, Ta; X?=?S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ?13?Å and ?7?Å for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.

Li, Yan; Kang, Jun; Li, Jingbo, E-mail: jbli@semi.ac.cn [State Key Laboratory of Superlattice and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083 (China); Tongay, Sefaattin; Wu, Junqiao [Department of Materials Science and Engineering, University of California, Berkeley, California 94720 (United States); Yue, Qu [College of Science, National University of Defense Technology, Changsha 410073 (China)

2013-11-07

22

Metal to semiconductor transition in metallic transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX2 (M = Nb, Ta; X = S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ˜13 Å and ˜7 Å for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.

Li, Yan; Tongay, Sefaattin; Yue, Qu; Kang, Jun; Wu, Junqiao; Li, Jingbo

2013-11-01

23

Understanding instabilites in early transition metal carbonitrides---A case study of MoC and MoN  

NASA Astrophysics Data System (ADS)

Rocksalt structure carbo/nitride systems, consisting of group IV, V, or VI transition-metal elements and C or N, have been studied extensively because of their extrodinary strength and high melting temperatures. In addition, several of them exhibit relatively high temperature superconductivity (e.g. NbN), phonon and elastic anomalies (e.g. MoN), as well as intrinsic substoichiometry for most of them. We are studying MoN (which is clearly unstable) and MoC (which seems to lie on the border between stability and instability). We have undertaken a comprehensive study of the electronic structure, elastic constants, and phonons of these two systems to gain insight into correlations between observed properties and electronic structure-related properties of carbo/nitride systems. Here we present results for the band structures, Fermi surfaces, generalized susceptibilites, elastic constants and some phonon modes of these materials, and we will discuss the relationship of these results to the observed instabilies.

Hart, Gus L. W.; Lu, Z. W.; Klein, Barry M.

1997-08-01

24

Dependence of the elastic properties of the early-transition-metal monoborides on their electronic structures: A density functional theory study  

NASA Astrophysics Data System (ADS)

We systematically investigated the crystal structure, stability, elastic properties, chemical bonding and electronic properties of the early-transition-metal monoborides (TMBs, where TM=Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Mo, and W) using the ab initio calculations based on the density functional theory. The results indicated that all 11 TMBs crystallized to a CrB-type structure are thermodynamically and mechanically stable. The elastic constants were calculated using the finite strain method. The correlation between the electronic structure and elastic properties was discussed. YB was found to have high machinability (B/C44=1.73) and low hardness (C44=43 GPa). The weak interaction between the interleaved yttrium planes and weak pd bonding resulted in the good machinability of YB.

Xu, Xuewen; Fu, Kun; Li, Lanlan; Lu, Zunming; Zhang, Xinghua; Fan, Ying; Lin, Jing; Liu, Guodong; Luo, Hongzhi; Tang, Chengchun

2013-06-01

25

Electrical Conductivity in Transition Metals  

ERIC Educational Resources Information Center

The aim of this "Science Note" is to describe how to test the electron-sea model to determine whether it accurately predicts relative electrical conductivity for first-row transition metals. In the electron-sea model, a metal crystal is viewed as a three-dimensional array of metal cations immersed in a sea of delocalised valence…

Talbot, Christopher; Vickneson, Kishanda

2013-01-01

26

Selenophene transition metal complexes  

SciTech Connect

This research shows that selenophene transition metal complexes have a chemistry that is similar to their thiophene analogs. Selenophene coordination has been demonstrated and confirmed by molecular structure in both the {eta}{sup 5}- and the {eta}{sup 1}(Se)-coordination modes. The reaction chemistry of selenophene complexes closely resembles that of the analogous thiophene complexes. One major difference, however, is that selenophene is a better donor ligand than thiophene making the selenophene complexes more stable than the corresponding thiophene complexes. The {sup 77}Se NMR chemical shift values for selenophene complexes fall within distinct regions primarily depending on the coordination mode of the selenophene ligand. In the final paper, the C-H bond activation of {eta}{sup 1}(S)-bound thiophenes, {eta}{sup 1}(S)-benzothiophene and {eta}{sup 1}(Se)-bound selenophenes has been demonstrated. The deprotonation and rearrangement of the {eta}{sup 1}(E)-bound ligand to the carbon bound L-yl complex readily occurs in the presence of base. Reprotonation with a strong acid gives a carbene complex that is unreactive towards nucleophilic attack at the carbene carbon and is stable towards exposure to air. The molecular structure of [Cp(NO)(PPh{sub 3})Re(2-benzothioenylcarbene)]O{sub 3}SCF{sub 3} was determined and contains a Re-C bond with substantial double bond character. Methyl substitution for the thienylcarbene or selenylcarbene gives a carbene that rearranges thermally to give back the {eta}{sup 1}(E)-bound complex. Based on these model reactions, a new mechanism for the H/D exchange of thiophene over the hydrodesulfurization catalyst has been proposed.

White, C.J.

1994-07-27

27

Calixarene supported transition metal clusters.  

E-print Network

??This thesis describes a series of calix[n]arene polynuclear transition metal and lanthanide complexes. Calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of… (more)

Taylor, Stephanie Merac

2013-01-01

28

Early metal bis(phosphorus-stabilised)carbene chemistry.  

PubMed

Since the discovery of covalently-bound mid- and late-transition metal carbenes there has been a spectacular explosion of interest in their chemistry, but their early metal counterparts have lagged behind. In recent years, bis(phosphorus-stabilised)carbenes have emerged as valuable ligands for metals across the periodic table, and their use has in particular greatly expanded covalently-bound early metal carbene chemistry. In this tutorial review we introduce the reader to bis(phosphorus-stabilised)carbenes, and cover general preparative methods, structure and bonding features, and emerging reactivity studies of early metal derivatives (groups 1-4 and the f-elements). PMID:21321726

Liddle, Stephen T; Mills, David P; Wooles, Ashley J

2011-05-01

29

Pressure-Induced Electronic Phase Transitions Transition Metal Oxides and Rare Earth Metals  

E-print Network

Pressure-Induced Electronic Phase Transitions in Transition Metal Oxides and Rare Earth Metals Metal Oxides and Rare Earth Metals by Brian Ross Maddox Electron correlation can affect profound changes transition in a transition metal monoxide. iv #12;The lanthanides (the 4f metals also known as rare-earths

Islam, M. Saif

30

R3Au(6+x)Al26T (R = Ca, Sr, Eu, Yb; T = early transition metal): a large family of compounds with a stuffed BaHg11 structure type grown from aluminum flux.  

PubMed

A collection of new quaternary intermetallic compounds with a cubic, stuffed BaHg(11) structure type has been synthesized by the combination of a divalent rare earth or alkaline earth metal R, an early transition metal T, and gold in an excess of molten aluminum. Structural characterization of these R(3)Au(6+x)Al(26)T compounds by powder and single crystal X-ray diffraction indicates that the unit cell varies with the radii of the early transition metal T and the rare earth/alkaline earth R as expected. The element T (where T = group 4, 5, 6, and 7 element) appears to be responsible for the stabilization of up to 43 different members of the R(3)Au(6+x)Al(26)T family of compounds. Varying amounts of disorder and trends in partial occupancies of the Au stuffed site--the site that is vacant in the parent compound BaHg(11)--are also indicated by the diffraction studies of this family of compounds. Magnetic susceptibility data reveals that the transition metal atoms in these materials do not possess local magnetic moments. For the magnetic rare earth containing materials, the europium compounds undergo a ferromagnetic transition at 10 K, and the ytterbium analogues show mixed valent behavior. Band structure calculations also support a mixed valent state for Yb in these compounds. PMID:19146424

Latturner, Susan E; Bilc, Daniel; Mahanti, S D; Kanatzidis, Mercouri G

2009-02-16

31

Transition metal sulfide loaded catalyst  

DOEpatents

A zeolite based catalyst for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C.sub.2 + hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

Maroni, Victor A. (Naperville, IL); Iton, Lennox E. (Downers Grove, IL); Pasterczyk, James W. (Westmont, IL); Winterer, Markus (Westmont, IL); Krause, Theodore R. (Lisle, IL)

1994-01-01

32

Transition metal sulfide loaded catalyst  

DOEpatents

A zeolite-based catalyst is described for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C[sub 2]+ hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

Maroni, V.A.; Iton, L.E.; Pasterczyk, J.W.; Winterer, M.; Krause, T.R.

1994-04-26

33

Transition metals in superheat melts  

NASA Technical Reports Server (NTRS)

A series of experiments with silicate melts doped with transition element oxides was carried out at atmospheric pressures of inert gas at temperatures exceeding liquidus. As predicted from the shape of fO2 buffer curves in T-fO2 diagrams the reducing conditions for a particular oxide-metal pair can be achieved through the T increase if the released oxygen is continuously removed. Experimental studies suggest that transition metals such as Cr or V behave as siderophile elements at temperatures exceeding liquidus temperatures if the system is not buffered by the presence of other oxide of more siderophile element. For example the presence of FeO prevents the reduction of Cr2O3. The sequence of decreasing siderophility of transition elements at superheat conditions (Mo, Ni, Fe, Cr) matches the decreasing degree of depletion of siderophile elements in mantle rocks as compared to chondrites.

Jakes, Petr; Wolfbauer, Michael-Patrick

1993-01-01

34

Organic-inorganic hybrid chains and layers constructed from copper-amine cations and early transition metal (Nb, Mo) oxyfluoride anions.  

PubMed

Under hydrothermal conditions in HF-containing media, the combination of early (Nb(5+), Mo(6+)) and late (Cu(2+)) transition metals with four different organic amine ligands yields eight novel hybrid compounds with chain and layer structures. The eight new compounds were synthesised from a systematic series of reactions and can be subdivided into four pairs, the topologies of which are essentially unique to each ligand, containing in each case a Cu-based cationic species, but alternately either [MoO(2)F(4)](2-) or [NbOF(5)](2-), in an isomorphous manner, as the anionic moiety. The overall structures of theses materials reflect the influences of the organo amine ligands. [Cu(C(2)H(4)N(4))(2)][MoO(2)F(4)].H(2)O () and [Cu(C(2)H(4)N(4))(2)][NbOF(5)].H(2)O () exhibit an infinite copper fluoride amine chain, while the anionic moieties act as 'decoration' to the chain. [Cu(C(3)H(4)N(2))(4)][MoO(2)F(4)] () and [Cu(C(3)H(4)N(2))(4)][NbOF(5)] () form infinite 1D chains with alternating cationic and anionic units. [Cu(2)F(2)(C(10)H(9)N(3))(2)][MoO(2)F(4)] () and [Cu(2)F(2)(C(10)H(9)N(3))(2)] [NbOF(5)] () contain a dimeric copper fluoride complex, bridged through anions to form a 1D infinite chain. [Cu(C(10)H(8)N(2))(2)(H(2)O)(2)][MoO(2)F(4)] () and [Cu(C(10)H(8)N(2))(2)(H(2)O)(2)][NbOF(5)] () contain a 2D interpenetrated network formed from the copper-4,4'-bipy complex, with open channels which accommodate the oxyfluoride anions via hydrogen bond interactions. PMID:19421630

Mahenthirarajah, Thushitha; Li, Yang; Lightfoot, Philip

2009-05-01

35

Approximating Metal-Insulator Transitions  

E-print Network

We consider quantum wave propagation in one-dimensional quasiperiodic lattices. We propose an iterative construction of quasiperiodic potentials from sequences of potentials with increasing spatial period. At each finite iteration step the eigenstates reflect the properties of the limiting quasiperiodic potential properties up to a controlled maximum system size. We then observe approximate metal-insulator transitions (MIT) at the finite iteration steps. We also report evidence on mobility edges which are at variance to the celebrated Aubry-Andre model. The dynamics near the MIT shows a critical slowing down of the ballistic group velocity in the metallic phase similar to the divergence of the localization length in the insulating phase.

C. Danieli; K. Rayanov; B. Pavlov; G. Martin; S. Flach

2014-05-06

36

Bonding and phase transitions in transition metal dichalcogenide layer compounds  

Microsoft Academic Search

The electronic structure of the transition metal dichalcogenide layer compounds is studied using hybridised orbitals appropriate to the transition metal atom coordination. Non-bonding orbitals on the transition metal atoms give the conduction electron bands which are responsible for the charge-density-wave phase transitions in the VB compounds, and the band structure of 1T- and 2H-TaS2 is found using tight binding. The

J. E. Inglesfield

1980-01-01

37

THE METAL-NON-METAL TRANSITION Cavendish Laboratory, Cambridge  

E-print Network

oxide, the material which in 1937 first made us realise that the Bloch-Wilson theory of metalsTHE METAL-NON-METAL TRANSITION N. F. MOTT Cavendish Laboratory, Cambridge RBsumB. -La transition mktal-non metal discutke dans cet article est celle qui apparait comme une condquence de l'knergie de

Boyer, Edmond

38

The metal-non-metal transition in compressed metal vapours  

NASA Astrophysics Data System (ADS)

Knowledge of the properties of hydrogen and helium and their mixtures, at temperatures and pressures prevailing in the giant planets is of considerable interest for planetary modelling. In the light of the unfavourable outlook for reliable measurements under these extreme conditions effort has been spent investigating the high-temperature high-pressure properties of fluid metals which are experimentally accessible in the laboratory and which might serve as models for compressed fluid hydrogen. The main emphasis of the paper is on the density dependence of the dynamic structure factor 0953-8984/10/49/026/img1 of liquid rubidium which reveals that a monoatomic-molecular transition occurs in the metal-non-metal transition region of the expanded liquid analogous to that suggested to occur in shock compressed hydrogen. Additional emphasis is on new results of the phase behaviour of dilute mixtures of helium in the near critical metal mercury.

Hensel, F.; Marceca, E.; Pilgrim, W. C.

1998-12-01

39

Synthesis of transition metal carbonitrides  

DOEpatents

Transition metal carbonitrides (in particular, titanium carbonitride, TiC.sub.0.5 N.sub.0.5) are synthesized by a self-propagating reaction between the metal (e.g., titanium) and carbon in a nitrogen atmosphere. Complete conversion to the carbonitride phase is achieved with the addition of TiN as diluent and with a nitrogen pressure .gtoreq.0.6 MPa. Thermodynamic phase-stability calculations and experimental characterizations of quenched samples provided revealed that the mechanism of formation of the carbonitride is a two-step process. The first step involves the formation of the nonstoichiometric carbide, TiC.sub.0.5, and is followed by the formation of the product by the incorporation of nitrogen in the defect-structure carbide.

Munir, Zuhair A. R. (Davis, CA); Eslamloo-Grami, Maryam (Davis, CA)

1994-01-01

40

Complexity in Transition Metal Oxides  

NASA Astrophysics Data System (ADS)

Recent computational results in the context of models for manganites and cuprates will be briefly discussed. It is argued that correlations in quenched disorder -- needed to mimic cooperative Jahn-Teller effects -- are important to have colossal magnetoresistance in 3D. A related recently discussed metal-insulator transition induced by disorder in a one-orbital model with cooperative phonons is intuitively explained [1]. In addition, it is argued that colossal effects should be far more common than currently known, and they may appear in cuprate superconductors as well [2]. [1] J. Burgy et al., cond-mat/0308456; C. Sen, G. Alvarez, and E. Dagotto, preprint. [2] See also Adriana Moreo, invited talk, March APS 04; G. Alvarez, M. Mayr et al., preprint.

Dagotto, Elbio; Alvarez, Gonzalo; Moreo, Adriana

2004-03-01

41

Transition Metal Dichalcogenides and Their Intercalates  

Microsoft Academic Search

The transition metal dichalcogenides form a group of layered, highly anisotropic compounds which exhibit interesting and unusual physical properties. Compounds formed from the Group Iv, Group V and Group VI transition metals and from sulphur or selenium have been the subject of a great deal of interest in the past few years because of the possibility of introducing foreign species

E. A. Marseglia

1983-01-01

42

Method for dry etching of transition metals  

DOEpatents

A method for dry etching of transition metals. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorous-containing .pi.-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/.pi.-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the .pi.-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the .pi.-acceptor ligand for forming the volatile transition metal/.pi.-acceptor ligand complex.

Ashby, Carol I. H. (Edgewood, NM); Baca, Albert G. (Albuquerque, NM); Esherick, Peter (Albuquerque, NM); Parmeter, John E. (Albuquerque, NM); Rieger, Dennis J. (Tijeras, NM); Shul, Randy J. (Albuquerque, NM)

1998-01-01

43

Interfacial properties of semiconducting transition metal chalcogenides  

NASA Astrophysics Data System (ADS)

This review is aimed at the correlation of structural and electronic properies of semiconducting transition metal chalcogenides with molecular surface processes and mechanisms in photoelectrochemistry, (photo)catalysis, geochemistry and hydrometallurgy. Layer-type, pyrite structured and transition metal cluster containing chalcogenides are selected as model systems to explain the principles involved. Special emphasis is given to the discussion of materials which involve transition metal d- states in the interfacial reaction pathways of holes and electrons. Since they initiate and control heterogeneous coordination chemistry at the surfaces they may provide the possibility of tailoring selective and catalytically demanding reactions. Examples of such mechanisms are presented and discussed in relation to surface properties involved.

Jaegermann, W.; Tributsch, H.

44

Phase transitions in the early universe  

NASA Astrophysics Data System (ADS)

I explore the theory and computation of early-Universe finite-temperature phase transitions involving scalar fields. I focus primarily on the electroweak phase transition, but some of the methods I develop are applicable to any scalar-field cosmological phase transition (such as the computation of the lifetime of zero-temperature metastable vacua). I begin by examining phase transition thermodynamics with many extra coupled degrees of freedom, finding that such transitions have the potential to produce large amounts of entropy and can significantly dilute the concentration of thermal relic species (e.g., dark matter). I then detail a novel algorithm for calculating instanton solutions with multiple dynamic scalar fields, and present a computational package which implements the algorithm and computes the finite-temperature phase structure. Next, I discuss theoretical and practical problems of gauge dependence in finite-temperature effective potentials, using the Abelian Higgs and Abelian Higgs plus singlet models to show the severity of the problem. Finally, I apply the aforementioned algorithm to the electroweak phase transition in the next-to-minimal supersymmetric standard model (NMSSM). My collaborators and I find viable regions of the NMSSM which contain a strongly first-order phase transition and large enough CP violation to support electroweak baryogenesis, evade electric dipole moment constraints, and provide a dark matter candidate which could produce the observed 130 GeV gamma-ray line observed in the galactic center by the Fermi Gamma-ray Space Telescope.

Wainwright, Carroll L.

45

Thermomechanical properties of 3d transition metals  

SciTech Connect

The authors have investigated the density variation of the Einstein temperatures and elastic constants of the 3d transition metals. In this respect they have employed the transition metal (TM) pair potentials involving the sp contribution with an appropriate exchange and correlation function, the d-band broadening contribution and the d-band hybridization term. These calculations are aimed at testing the TM pair potentials in generating the quasilocal and local thermomechanical properties.

Karaoglu, B.; Rahman, S.M.M. (Sultan Qaboos Univ., Muscat (Oman). Dept. of Physics)

1994-05-15

46

Orbital Physics in Transition-Metal Oxides  

Microsoft Academic Search

An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions

Y. Tokura; N. Nagaosa

2000-01-01

47

Quantum Criticality in Transition-Metal Oxides  

Microsoft Academic Search

We report on experiments of the bulk susceptibility ?(T), heat capacity C(T)\\/T, resistivity ?(T) and nuclear resonances (NMR and NQR) in order to review evidence of quantum critical behaviour in some metallic transition-metal\\u000a oxides. In analogy to the conventional 4f- and 5f-electron based heavy-fermion compounds, the prerequisites of quantum criticality, i.e. a magnetic phase transition at T=0 accompanied by non-Fermi

N. Büttgen; H.-A. Krug von Nidda; W. Kraetschmer; A. Günther; S. Widmann; S. Riegg; A. Krimmel; A. Loidl

2010-01-01

48

Continuous and discontinuous metal-insulator transitions  

Microsoft Academic Search

In non-crystalline systems a continuous metal-insulator transition of Andersontype is frequently observed when the Fermi energy moves through a mobility edge. In crystalline systems, on the other hand, the activation energy for conduction should change discontinuously at the transition. The aim of this paper is to enquire under what conditions a discontinuous transition can occur in non-crystalline systems. It clearly

N. F. Mott

1978-01-01

49

Magnetic susceptibility of metallic transition-metal dichalcogenides  

Microsoft Academic Search

A tight-binding model has been used to estimate the anisotropy of the magnetic susceptibility of the metallic transition-metal dichalcogenides. The calculated anisotropies obtained are in qualitative agreement with experiment and can be interpreted in terms of the electronic structure of these compounds. It is expected that these results will make possible a more detailed interpretation of the changes in magnetic

A. H. MacDonald; D. J. W. Geldart

1981-01-01

50

film across metal to insulator transition  

NASA Astrophysics Data System (ADS)

We have proposed a method to probe metal to insulator transition in VO2 measuring photoluminescence response of colloidal quantum dots deposited on the VO2 film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO2 across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data.

Kuznetsov, Sergey N.; Cheremisin, Alexander B.; Stefanovich, Genrikh B.

2014-11-01

51

Microwave-assisted synthesis of transition metal phosphide  

DOEpatents

A method of synthesizing transition metal phosphide. In one embodiment, the method has the steps of preparing a transition metal lignosulfonate, mixing the transition metal lignosulfonate with phosphoric acid to form a mixture, and subjecting the mixture to a microwave radiation for a duration of time effective to obtain a transition metal phosphide.

Viswanathan, Tito

2014-12-30

52

Method of boronizing transition metal surfaces  

DOEpatents

A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB[sub 2], or CrB[sub 2]. A transition metal to be coated is immersed in the melt at a temperature of no more than 700 C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface. 4 figs.

Koyama, Koichiro; Shimotake, Hiroshi.

1983-08-16

53

Charge Density Wave Transitions in the Transition Metal Dichalcogenides  

Microsoft Academic Search

A microscopic Hamiltonian descriptive of the charge density wave (C.D.W.) transitions occurring in the 2H transition metal dichalcogenides is presented. From existing band structure calculations of these highly two dimensional compounds, the essential electronic feature is proposed to be the six equivalent (GAMMA)-K saddle points of the conduction band which in the two dimensional limit produce a logarithmic Van Hove

Gary Kingsbury Scott

1983-01-01

54

Atomic layer deposition of transition metals  

NASA Astrophysics Data System (ADS)

Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapours of two chemical reactants. ALD processes have been successfully demonstrated for many metal compounds, but for only very few pure metals. Here we demonstrate processes for the ALD of transition metals including copper, cobalt, iron and nickel. Homoleptic N,N'-dialkylacetamidinato metal compounds and molecular hydrogen gas were used as the reactants. Their surface reactions were found to be complementary and self-limiting, thus providing highly uniform thicknesses and conformal coating of long, narrow holes. We propose that these ALD layers grow by a hydrogenation mechanism that should also operate during the ALD of many other metals. The use of water vapour in place of hydrogen gas gives highly uniform, conformal films of metal oxides, including lanthanum oxide. These processes should permit the improved production of many devices for which the ALD process has previously not been applicable.

Lim, Booyong S.; Rahtu, Antti; Gordon, Roy G.

2003-11-01

55

Antiferromagnetism in 4 d transition metals  

SciTech Connect

Total-energy band calculations employing a fixed-spin-moment procedure and the augmented-spherical-wave method are used to study the volume dependences and existence ranges of antiferromagnetic behavior in the 4{ital d} transition metals constrained to cubic lattices. At expanded volumes we find stable antiferromagnetic solutions of the Kohn-Sham equations for bcc niobium, molybdenum, and technetium, and for fcc technetium, and metastable antiferromagnetic solutions for fcc ruthenium. We find no stable antiferromagnetic solutions for fcc rhodium or palladium. Comparisons are made with the occurrence of antiferromagnetism in the 3{ital d} transition metals.

Moruzzi, V.L.; Marcus, P.M. (IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, NY (USA))

1990-12-01

56

Wettability of transition metal oxide surfaces  

Microsoft Academic Search

The wettability of transition metal oxide surfaces (TiO2, ZrO2 and HfO2) by water was investigated by measuring the contact angles using the sessile drop method. The oxide surfaces were prepared by thermal oxidation of polished metal foils of Ti, Zr and Hf, which had been polished to different levels of surface roughness. The surfaces were also characterized by optical microscopy,

Aiguo Feng; Benjamin J McCoy; Zuhair A Munir; Domenick Cagliostro

1998-01-01

57

Upconversion Processes in Transition Metal and Rare Earth Metal Systems  

Microsoft Academic Search

The design and characterization of new luminescent materials is an active area of research. Here we present several current\\u000a topics in the area of upconversion by transition-metal and rare-earth-metal doped halide lattices. Following introduction\\u000a to the necessary background material related to upconversion mechanisms and kinetics, a series of topics are discussed which\\u000a illustrate some key areas of developing interest in

Daniel R. Gamelin; Hans U. Güdel

58

Electronic properties of intercalation complexes of the transition metal dichalcogenides  

Microsoft Academic Search

Intercalation of the layer type transition metal dichalcogenides by a variety of organic molecules, alkali metals, or `3d' transition metals, provides a powerful way to finely tune the electron occupation of the relatively narrow `d' bands met in these solids. These transition metal dichalcogenides are highly anisotropic solids, sometimes referred to as `two-dimensional' solids, and the intercalant molecules which are

R. H. Friend; A. D. Yoffe

1987-01-01

59

Electronic properties of intercalation complexes of the transition metal dichalcogenides  

Microsoft Academic Search

Intercalation of the layer type transition metal dichalcogenides by a variety of organic molecules, alkali metals, or ‘3d’ transition metals, provides a powerful way to finely tune the electron occupation of the relatively narrow ‘d’ bands met in these solids. These transition metal dichalcogenides are highly anisotropic solids, sometimes referred to as ‘two-dimensional’ solids, and the intercalant molecules which are

R. H. Friend; A. D. Yoffe

1987-01-01

60

New intercalation compounds of transition metal dichalcogenides  

Microsoft Academic Search

In this paper we describe the preparation and characterization of ethylenediamine-intercalated 6R-TaS2 and octylamine-intercalated 2H-TaS2. We also describe attempts to intercalate various electron acceptor species into transition metal dichalcogenides.

E. Figueroa; J. W. Brill; J. P. Selegue

1996-01-01

61

OH-transition metal bonding  

NASA Technical Reports Server (NTRS)

The bonding in both CuOH and AgOH has a covalent component that leads to a bent structure. The larger electrostatic stabilization in CuOH leads to a larger D(e) (2.83 eV) compared with AgOH (2.20 eV). Using Ni5OH to model chemisorption of OH on a Ni surface, it is found that OH adsorption in the fourfold hollow of Ni(100) leads to an OH normal to the surface, while adsorption directly above a Ni atom leads to a tilted OH. These qualitative Ni5OH calculations allow for speculation on the observed variation of OH on metal surfaces.

Bauschlicher, Charles W., Jr.

1986-01-01

62

Early Main Group Metal Catalysis: How Important is the Metal?  

PubMed

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal-alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca(2+) . The role of the metal was evaluated by a study using the metal-free catalysts: [Ph2 N(-) ][Me4 N(+) ] and [Ph3 C(-) ][Me4 N(+) ]. These "naked" amides and carbanions can act as catalysts in the conversion of activated double bonds (C?O and C?N) in the hydroamination of Ar?N?C?O and R?N?C?N?R (R=alkyl) by Ph2 NH. For the intramolecular hydroamination of unactivated C?C bonds in H2 C?CHCH2 CPh2 CH2 NH2 the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study. PMID:25376952

Penafiel, Johanne; Maron, Laurent; Harder, Sjoerd

2014-11-01

63

Polymer nanocomposites based on transition metal ion modified organoclays  

E-print Network

Polymer nanocomposites based on transition metal ion modified organoclays Pranav Nawani a , Priya: Transition metal ion; Organoclay; Nanocomposite 1. Introduction Thermoplastic nanocomposites with improved; accepted 4 December 2006 Available online 3 January 2007 Abstract A unique class of nanocomposites

Frenkel, Anatoly

64

(Electronic structure and reactivities of transition metal clusters)  

SciTech Connect

The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.

Not Available

1992-01-01

65

Identification of non-classical C-H···M interactions in early and late transition metal complexes containing the CH(ArO)3 ligand.  

PubMed

The fully optimised DFT structure of the d(0) complex [{CH(ArO)3}Ti(NEt2)] (2) at the B3LYP level compares well with the distorted tetrahedral geometry shown by the X-ray crystal structure. QTAIM analysis of the electron density associated with the C-H···Ti interaction shows a well defined bond critical point, a bond path between the hydrogen and titanium centres and a negative value for the energy density indicative of covalency. A natural bond orbital (NBO) picture of the interaction shows that the C-H ? bond electron density donates to a d hybrid orbital on the metal in a linear fashion. Calculated IR and NMR data for the components of the interaction are consistent with experiment. The computed structures for [{CH(ArO)3}Ti(OPh)] (3), [{CH(ArO)3}Zr(NEt2)] (4), [{CH(ArO)3}Hf(NEt2)] (5), show tetrahedral geometries and QTAIM and NBO properties similar to (2). [{CH(ArO)3}Mo(NEt2)] (6) shows distortion of the tripodal ligand and a reduced C-H···M bond angle with properties more consistent with a C-H···M side-on donor interaction. In [{CH(ArO)3}Fe(NEt2)] (7) the C-H···M bond angle is linear and involves a donor interaction. An energy minimised structure maintaining the three fold coordination to the tripodal ligand was not obtained for [{CH(ArO)3}Ni(NEt2)](2-) but changing from a diethyl amide ligand to phenolato gave energy minimised [{CH(ArO)3}Ni(OPh)](2-) (8). This structure shows a distorted square planar geometry with a substantially bent phenoxo ligand and a near linear C-H···M covalent interaction with donor and back bonding properties. The work shows that linear C-H···M interactions can have both agostic and weak hydrogen bond-like covalency. PMID:23788028

Lein, Matthias; Harrison, John A; Nielson, Alastair J

2013-08-14

66

THEORY OF MOTTTRANSITION : APPLICATION S TO TRANSITION METAL OXIDES  

E-print Network

125 THEORY OF MOTTTRANSITION : APPLICATION S TO TRANSITION METAL OXIDES M. CYROT Institut Laue discuss the relevance of the theory to transition metal oxides. The main qualitative features in transition metal oxides. These oxides form a very interesting class of materials [1]. Their electrical

Paris-Sud XI, Université de

67

MATERIALS THEORY From transition metal oxides to cosmic strings  

E-print Network

MATERIALS THEORY From transition metal oxides to cosmic strings (and how electronic structure Transition metal oxides Thursday, September 20, 2012 6D-MATL / Materials Theory Compounds containing and competing interactions! Energy scale ~ 1eV #12;MATERIALS THEORY Where do transition metal oxides fit

68

Topological crystalline insulators in transition metal oxides.  

PubMed

Topological crystalline insulators possess electronic states protected by crystal symmetries, rather than time-reversal symmetry. We show that the transition metal oxides with heavy transition metals are able to support nontrivial band topology resulting from mirror symmetry of the lattice. As an example, we consider pyrochlore oxides of the form A2M2O7. As a function of spin-orbit coupling strength, we find two Z2 topological insulator phases can be distinguished from each other by their mirror Chern numbers, indicating a different topological crystalline insulators. We also derive an effective k·p Hamiltonian, similar to the model introduced for Pb(1-x)Sn(x)Te, and discuss the effect of an on-site Hubbard interaction on the topological crystalline insulator phase using slave-rotor mean-field theory, which predicts new classes of topological quantum spin liquids. PMID:25167290

Kargarian, Mehdi; Fiete, Gregory A

2013-04-12

69

Metal-insulator transition in vanadium dioxide  

Microsoft Academic Search

The basic physical parameters which govern the metal-insulator transition in vanadium dioxide are determined through a review of the properties of this material. The major importance of the Hubbard intra-atomic correlation energy in determining the insulating phase, which was already evidenced by studies of the magnetic properties of V1-xCrxO2 alloys, is further demonstrated from an analysis of their electrical properties.

A. Zylbersztejn; N. F. Mott

1975-01-01

70

Transition metal dichalcogenide\\/polymer nanocomposites  

Microsoft Academic Search

Single molecular layers of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2 and WS2, have been formed in suspension in water by lithium intercalation of crystalline powders and then exfoliation in water. The two-dimensionality of such systems is readily identified using powder X-ray diffraction, where the strong asymmetric shape of the (hk0) peaks and the absence of (00?) peaks and

D. Yang; P. Westreich; R. F. Frindt

1999-01-01

71

Surface resonances at transition metal dichalcogenide heterostructures  

Microsoft Academic Search

Layered transition metal dichalcogenides do generally not exhibit characteristic electronic surface states localized perpendicular to the layers. Employing van der Waals epitaxy together with angle-resolved photoemission spectroscopy we show how surface-layer-derived electronic states can be generated on these materials. For a heterojunction consisting of one HfS2 epilayer adsorbed on bulk WSe2, purely two-dimensional behavior as well as three-dimensional coupling of

C. Kreis; S. Werth; R. Adelung; L. Kipp; M. Skibowski; D. Voß; P. Krüger; A. Mazur; J. Pollmann

2002-01-01

72

Single-layer transition metal sulfide catalysts  

DOEpatents

Transition Metal Sulfides (TMS), such as molybdenum disulfide (MoS.sub.2), are the petroleum industry's "workhorse" catalysts for upgrading heavy petroleum feedstocks and removing sulfur, nitrogen and other pollutants from fuels. We have developed an improved synthesis technique to produce SLTMS catalysts, such as molybdenum disulfide, with potentially greater activity and specificity than those currently available. Applications for this technology include heavy feed upgrading, in-situ catalysis, bio-fuel conversion and coal liquefaction.

Thoma, Steven G. (Albuquerque, NM)

2011-05-31

73

Effect of Transition Metal Impurities on Superconductivity  

Microsoft Academic Search

The existing experimental results on the effect of transition metal impurities on superconductivity in several alloy systems were compared with theories based on two different models, the Anderson model and the s-d model. It was found that the theory of Müller-Hartmann and Zittartz (s-d model) reproduces the experimental results in Kondo alloys with TK\\/Tco≲1 fairly well if TK of the

Shigeru Takayanagi; Tadashi Sugawara

1975-01-01

74

Smooth phase transitions in the early universe  

E-print Network

We apply a quadratic teleparallel torsion scalar of the $f(T)=T+\\alpha T^{2}$ field equations to the flat FRW model. We assume two perfect fluid components, the matter component has a fixed equation of state (EoS), while the torsion component has a dynamical EoS. We obtain an effective scale factor allowing an early stage of an inflationary vacuum, while it pushes the inflation to end naturally at later stage turning the universe to a matter dominant phase. The model represents an alternative to inflation models. However, it does not require slow roll conditions to end the inflation phase. We perform a standard cosmological study to examine the cosmic evolution. In addition, we study the effective EoS. The study shows consistent results confirming a smooth phase transition from inflation to matter dominant universe. We consider the case when the torsion is made of a scalar field. This treatment enables us to study the quadratic effect of $f(T)$ on the potential patterns of the scalar field. At the limit $\\alpha \\rightarrow 0$ the potential is dominated by the kinetic term and coincides with the quadratic inflation. Both cosmological and scalar field analysis show consistent results.

G. G. L. Nashed; W. El Hanafy; Sh. Kh. Ibrahim

2014-11-09

75

Corrosion behavior of mesoporous transition metal nitrides  

SciTech Connect

Transition metal nitrides (TMN) have many desirable characteristics such as high hardness and good thermal stability under reducing conditions. This work reports an initial survey of the chemical stability of mesoporous TMNs (TM=Nb, V, Cr and Ti) in water at 80 °C at neutral, acidic and alkaline pH. The mesoporous TMNs had specific surface areas of 25–60 m{sup 2}/g with average pore sizes ranging from 10 to 50 nm. The high surface areas of these materials enhance the rate of corrosion per unit mass over that of a bulk material, making detection of corrosion much easier. The products were characterized by Rietveld refinement of powder X-ray diffraction (PXRD) patterns and by scanning electron microscopy (SEM). Several nitrides have corrosion rates that are, within error, not distinguishable from zero (±1 Å/day). Of the nitrides examined, CrN appears to be the most corrosion resistant under acidic conditions. None of the nitrides studied are corrosion resistant under alkaline conditions. - Graphical abstract: Corrosion behavior of mesoporous transition metal nitrides (TM=Nb, V, Cr and Ti) in acidic and alkaline solutions at 80 °C for 2 weeks. Display Omitted - highlights: • Corrosion rates of mesoporous transition metal nitrides in aqueous solution is reported. • The mesoporous TMNs had surface areas of 25–60 m{sup 2}/g. • CrN is the most corrosion resistant under the conditions studied.

Yang, Minghui, E-mail: m.yang@cornell.edu [Department of Chemistry, Cornell University, Ithaca 14853-1301, NY (United States); Allen, Amy J.; Nguyen, Minh T. [Department of Chemistry, Cornell University, Ithaca 14853-1301, NY (United States); Ralston, Walter T. [College of Chemistry, University of California, Berkeley 94720-1460, CA (United States); MacLeod, Michelle J. [Department of Chemistry, Massachusetts Institute of Technology, Cambridge 02139-4307, MA (United States); DiSalvo, Francis J., E-mail: fjd3@cornell.edu [Department of Chemistry, Cornell University, Ithaca 14853-1301, NY (United States)

2013-09-15

76

Atomic layer deposition of transition metals.  

PubMed

Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapours of two chemical reactants. ALD processes have been successfully demonstrated for many metal compounds, but for only very few pure metals. Here we demonstrate processes for the ALD of transition metals including copper, cobalt, iron and nickel. Homoleptic N,N'-dialkylacetamidinato metal compounds and molecular hydrogen gas were used as the reactants. Their surface reactions were found to be complementary and self-limiting, thus providing highly uniform thicknesses and conformal coating of long, narrow holes. We propose that these ALD layers grow by a hydrogenation mechanism that should also operate during the ALD of many other metals. The use of water vapour in place of hydrogen gas gives highly uniform, conformal films of metal oxides, including lanthanum oxide. These processes should permit the improved production of many devices for which the ALD process has previously not been applicable. PMID:14578877

Lim, Booyong S; Rahtu, Antti; Gordon, Roy G

2003-11-01

77

Synthesis and X-ray characterization of 3D transition metal intercalates of transition metal dichalcogenides and their superlattices  

Microsoft Academic Search

New superlattices of 3d transition metal intercalates of transition metal dichalcogenides have been synthesized by deposition and annealing of layered reactants. Sheets of atoms are the building blocks of transition metal dichalcogenides. Since the inter-sheet bonding is weak, guest atoms may be introduced between the sheets (intercalation) and sheets of differing atomic species may be stacked one atop the other

Thomas Abbott Hughes

1998-01-01

78

Metal-insulator transition in holography  

E-print Network

We exhibit an interaction-driven metal-insulator quantum phase transition in a holographic model. Use of a helical lattice enables us to break translation invariance while preserving homogeneity. The metallic phase is characterized by a sharp Drude peak and a d.c. resistivity that increases with temperature. In the insulating phase the Drude spectral weight is transferred into a `mid-infrared' peak and to energy scales of order the chemical potential. The d.c. resistivity now decreases with temperature. In the metallic phase, operators breaking translation invariance are irrelevant at low energy scales. In the insulating phase, translation symmetry breaking effects are present at low energies. We find the near horizon extremal geometry that captures the insulating physics.

Aristomenis Donos; Sean A. Hartnoll

2013-01-19

79

Catabolism of hyaluronan: involvement of transition metals.  

PubMed

One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes.Eight transition metals - V(23), Mn(25), Fe(26), Co(27), Ni(28), Cu(29), Zn(30), and Mo(42) - naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint.In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed. PMID:21217859

Soltés, Ladislav; Kogan, Grigorij

2009-12-01

80

Synthesis of arsenic transition metal sulfides and metal arsenides  

SciTech Connect

One of the chief problems in upgrading shale oil is the presence of inherent arsenic which is known to poison downstream catalysts. Highly dispersed transition metal sulfides formed in situ from the decomposition of dithiocarbamate (DTC) complexes of transition metals show excellent potential as dearsenation agents. The authors have studied the reaction of these sulfides with various arsenic compositions and characterized the metal arsenides and arsenic metal sulfides formed as well as the ease of their formation. Thus, the reaction of bis(butyldithiocarbamato)Ni, (NiBuDTC) with model compounds was very facile and gave NiAs, NiAsS, and NiAs2=xSx. In general the effectiveness of the sulfides for dearsenation followed the sequence Ni>Mo{much_gt}Co, while iron sulfides were totally ineffective. Based upon these results, tests were run in autoclaves (as well as a fixed-bed flow-through unit) with NiBuDTC and shale oil having 73 ppm inherent As. Under optimum conditions, dearsenation down to les than 1 ppm was obtained.

Singhal, G.H.; Brown, L.D.; Ryan, D.F. [Exxon Research & Development Labs, Baton Rouge, LA (United States)

1993-12-31

81

Routines and Transitions: A Guide for Early Childhood Professionals  

ERIC Educational Resources Information Center

In early childhood settings, children spend over 50 percent of their time on handwashing, dressing, napping, and other routines and transitions. "Routines and Transitions" is a guide to help turn these routine daily activities into learning experiences. By using transitions wisely, providers not only help children develop skills, but also run a…

Malenfont, Nicole

2006-01-01

82

Transitions for Young Children: Creating Connections across Early Childhood Systems  

ERIC Educational Resources Information Center

Smooth early childhood transitions are key to ensuring positive outcomes for young children the world over--but in today's fragmented early education systems, it's difficult to ensure continuity among programs and services. Early childhood professionals will help change that with this book, the first to propose a comprehensive, practical framework…

Kagan, Sharon Lynn, Ed.; Tarrant, Kate, Ed.

2010-01-01

83

Cosmological Consequences of QCD Phase Transition(s) in Early Universe  

SciTech Connect

We discuss the cosmological consequences of QCD phase transition(s) on the early universe. We argue that our recent knowledge about the transport properties of quark-gluon plasma (QGP) should thraw additional lights on the actual time evolution of our universe. Understanding the nature of QCD phase transition(s), which can be studied in lattice gauge theory and verified in heavy ion experiments, provides an explanation for cosmological phenomenon stem from early universe.

Tawfik, A. [ECTP, Egyptian Center for Theoretical Physics, MTI Modern University, Mukattam Cairo (Egypt)

2009-04-17

84

Magnetic correlations in doped transition metal oxides  

SciTech Connect

The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y{sub 2{minus}x}Ca{sub x}BaNiO{sub 5}, the two-dimensional cuprate superconductors La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3} O{sub 6+x}, and the classical three-dimensional ``Mott-Hubbard`` system V{sub 2{minus}y}O{sub 3}.

Aeppli, G. [AT and T Bell Labs., Murray Hill, NJ (United States); Bao, W.; Broholm, C. [Johns Hopkins Univ., Baltimore, MD (United States)] [and others

1995-02-15

85

Critical properties of metal-insulator transition in low dimensions  

Microsoft Academic Search

Anomalous metallic states near the Mott insulator are analyzed in terms of the critical scaling toward the metal-insulator transition. Continuous transitions from a metal to the Mott insulator are characterized either by the mass divergence as in the bi-component systems or the vanishing carrier number as in the valence bond systems. A set of critical exponents in these two cases

Masatoshi Imada

1995-01-01

86

Two-dimensional transition metal carbides.  

PubMed

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior. PMID:22279971

Naguib, Michael; Mashtalir, Olha; Carle, Joshua; Presser, Volker; Lu, Jun; Hultman, Lars; Gogotsi, Yury; Barsoum, Michel W

2012-02-28

87

Metal Insulator transition in Vanadium Dioxide  

NASA Astrophysics Data System (ADS)

MAR12-2011-000262 Abstract Submitted for the MAR12 Meeting of The American Physical Society Sorting Category: 03.9 (T) On the metal-insulator-transition in vanadium dioxide AZITA JOVAINI, SHIGEJI FUJITA, University at Buffalo, SALVADOR GODOY, UNAM, AKIRA SUZUKI, Tokyo University of Science --- Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 340 K with the structural change from tetragonal to monoclinic crystal. The conductivity _/ drops at MIT by four orders of magnitude. The low temperature monoclinic phase is known to have a lower ground-state energy. The existence of the k-vector k is prerequisite for the conduction since the k appears in the semiclassical equation of motion for the conduction electron (wave packet). The tetragonal (VO2)3 unit is periodic along the crystal's x-, y-, and z-axes, and hence there is a three-dimensional k-vector. There is a one-dimensional k for a monoclinic crystal. We believe this difference in the dimensionality of the k-vector is the cause of the conductivity drop. Prefer Oral Session X Prefer .

Jovaini, Azita; Fujita, Shigeji; Suzuki, Akira; Godoy, Salvador

2012-02-01

88

Plasmons in metallic monolayer and bilayer transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We study the collective electronic excitations in metallic single-layer and bilayer transition metal dichalcogenides (TMDCs) using time dependent density functional theory in the random phase approximation. For very small momentum transfers (below q?0.02 Å-1), the plasmon dispersion follows the q behavior expected for free electrons in two dimensions. For larger momentum transfer, the plasmon energy is significantly redshifted due to screening by interband transitions. At around q?0.1 Å-1, the plasmon enters the dissipative electron-hole continuum and the plasmon dispersions flatten out at an energy around 0.6-1.1 eV, depending on the material. Using bilayer NbSe2 as example, we show that the plasmon modes of a bilayer structure take the form of symmetric and antisymmetric hybrids of the single-layer modes. The spatially antisymmetric mode is rather weak with a linear dispersion tending to zero for q=0, while the energy of the symmetric mode follows the single-layer mode dispersion with a slight blue shift.

Andersen, Kirsten; Thygesen, Kristian S.

2013-10-01

89

An analytic model for phase transitions in the early universe  

Microsoft Academic Search

A simplified model of the very early universe is presented for studying the effects of the GUT phase transition on cosmic evolution and vice versa. An exact description of a strongly first order phase transition is given. Both inflationary and non-inflationary solutions are obtained. The cosmological scale factor and the temperature are determined and graphically depicted. The self-consistency of the

L. Diosi; B. Lukacs; B. Keszthelyi; G. Paal

1985-01-01

90

Electromagnetic Generation of Ultrasonic Waves in 3-d Transition Metals  

Microsoft Academic Search

Ultrasonic waves at megahertz frequencies can be generated in a metal by rf excitation of a coil placed next to the surface of the metal in the presence of a static magnetic field. We applied this technique to 3-d transition metals. The observed results differ from those obtained previously from nonmagnetic metals. When the static field was normal to the

M. Hanabusa; T. Kushida; J. C. Murphy

1974-01-01

91

Transition Metal Phosphide Hydroprocessing Catalysts: A review  

SciTech Connect

The diminishing quality of oil feedstocks coupled with increasingly more stringent environmental regulations limiting the content of sulfur in transportation fuels have given rise to a need for improved hydroprocessing technology. This review begins with a summary of the major improvements in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts and processes that have been reported in recent years. It then describes a new class of hydroprocessing catalysts, the transition metal phosphides, which have emerged as a promising group of high-activity, stable catalysts. The phosphides have physical properties resembling ceramics, so are strong and hard, yet retain electronic and magnetic properties similar to metals. Their crystal structures are based on trigonal prisms, yet they do not form layered structures like the sulfides. They display excellent performance in HDS and HDN, with the most active phosphide, Ni{sub 2}P, having activity surpassing that of promoted sulfides on the basis of sites titrated by chemisorption (CO for the phosphides, O{sub 2} for the sulfides). In the HDS of difficult heteroaromatics like 4,6-dimethyldibenzothiophene Ni{sub 2}P operates by the hydrogenation pathway, while in the HDN of substituted nitrogen compounds like 2-methylpiperidine it carries out nucleophilic substitution. The active sites for hydrogenation in Ni{sub 2}P have a square pyramidal geometry, while those for direct hydrodesulfurization have a tetrahedral geometry. Overall, Ni{sub 2}P is a promising catalyst for deep HDS in the presence of nitrogen and aromatic compounds.

Oyama, S.; Gott, T; Zhao, H; Lee, Y

2009-01-01

92

Tunable dielectric properties of transition metal dichalcogenides.  

PubMed

Since discovery of graphene, layered materials have drawn considerable attention because of their possible exfoliation into single and multilayer 2D sheets. Because of strong surface effects, the properties of these materials vary drastically with the number of layers in a sheet. We have performed first-principles density functional based calculations to evaluate the electron energy loss spectrum (EELS) of bulk, monolayer, and bilayer configurations of several transition metal dichalcogenides, which include semiconducting as well as metallic compounds. Our investigation shows that the peaks in the EELS spectra move toward larger wavelengths (red shift) with the decrease in number of layers. The ? plasmon peak shifts slightly by 0.5-1.0 eV, while a significant shift of around 5.5-13.0 eV is obtained for ? + ? plasmon, when exfoliated from bulk to single-layer. This underscores the importance of the interlayer coupling on the loss spectra and the dielectric properties. Our results are found to be in very good agreement with the recent measurements performed by Coleman et al. (Science2011, 331, 568). PMID:21707067

Johari, Priya; Shenoy, Vivek B

2011-07-26

93

Transition metal oxide nanowires synthesized by heating metal substrates  

SciTech Connect

Highlights: {center_dot} This paper describes a simple and general method to synthesize 3d metal oxide nanowires. {center_dot} Self-catalysis growth mechanism was proposed to explain the growth of the nanowires. {center_dot} The temperature range for the growth of nanowires was estimated by taking into account the Gibbs free energy of reaction. {center_dot} This synthesis approach could be applied to synthesize other one-dimensional structures, such as FeSe and Bi{sub 2}Te{sub 3} nanowires. -- Abstract: Here we reported a simple method to synthesize transition metal oxide nanowires. Copper oxide (CuO), zinc oxide (ZnO), and cobalt oxide (Co{sub 3}O{sub 4}) nanowires were synthesized by heating the copper, zinc, and cobalt substrates under atmosphere condition. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the morphology and microstructure of the nanowires. According to our experimental results, self-catalysis growth mechanism was proposed to explain the growth of the nanowires. The temperature window for the growth of nanowires was estimated by taking into account the Gibbs free energy of reaction. The synthesis approach observed in our experiment could be applied to synthesize other one-dimensional structures, such as FeSe and Bi{sub 2}Te{sub 3} nanowires.

Yan, Hui; Sun, Yi [Department of Physics, Beijing Normal University, 100875 Beijing (China)] [Department of Physics, Beijing Normal University, 100875 Beijing (China); He, Lin, E-mail: helin@bnu.edu.cn [Department of Physics, Beijing Normal University, 100875 Beijing (China)] [Department of Physics, Beijing Normal University, 100875 Beijing (China); Nie, Jia-Cai [Department of Physics, Beijing Normal University, 100875 Beijing (China)] [Department of Physics, Beijing Normal University, 100875 Beijing (China)

2011-11-15

94

Critical Point and Metal-Nonmetal Transition in Expanded Liquid Metals  

Microsoft Academic Search

For most metals the two phases of the liquid-gas transition are metallic in a region around the critical point. A method for determining the critical data of expanded liquid metals under such conditions has been published recently. Mercury, however, is a well-known exception with the metal-nonmetal transition inside the liquid phase. Mercury is no longer a metal at the critical

Helmut Hess; Hartmut Schneidenbach; Andreas Kloss

1999-01-01

95

Characterization of electrically conductive transition metal dichalcogenide lubricant films  

Microsoft Academic Search

Groups VB and VIB transition metal dichalcogenides with layered structures, are intrinsic solid lubricants, and constitute a class of materials with unique and unusual properties based on their extreme anisotropy. The primary objective of this investigation was to conduct a comprehensive study on the tribological and electrical properties of burnished and sputtered transition metal dichalcogenide films, and characterize the performance

Harish C. Waghray

1997-01-01

96

Trends in Ionization Energy of Transition-Metal Elements  

ERIC Educational Resources Information Center

A rationale for the difference in the periodic trends in the ionization energy of the transition-metal elements versus the main-group elements is presented. The difference is that in the transition-metal elements, the electrons enter an inner-shell electron orbital, while in the main-group elements, the electrons enter an outer-shell electron…

Matsumoto, Paul S.

2005-01-01

97

Nanostructured transition metal oxides useful for water oxidation catalysis  

DOEpatents

The present invention provides for a composition comprising a nanostructured transition metal oxide capable of oxidizing two H.sub.2O molecules to obtain four protons. In some embodiments of the invention, the composition further comprises a porous matrix wherein the nanocluster of the transition metal oxide is embedded on and/or in the porous matrix.

Frei, Heinz M; Jiao, Feng

2013-12-24

98

Superconductivity in carbon nanotubes coupled to transition metal atoms  

NASA Astrophysics Data System (ADS)

The electronic structures of zigzag and armchair single-walled carbon nanotubes interacting with a transitional-metal atomic nanowire of Ni have been determined. The Ni nanowire creates a large electron density of states (DOS) at the Fermi energy. The dependence of the enhanced DOS on the spin state and positioning of the transition metal wire (inside or outside the nanotube) is studied. Preliminary estimates of the electron-phonon interaction suggest that such systems may have a superconducting transition temperature of ~ 10 50 K. The signs of superconductivity seen in "ropes" of nanotubes may also be related to the effect of intrinsic transition metal impurities.

Tit, N.; Dharma-wardana, M. W. C.

2003-05-01

99

Nutritional immunity: transition metals at the pathogen-host interface  

PubMed Central

Transition metals occupy an essential niche in biological systems. Their electrostatic properties stabilize substrates or reaction intermediates in the active sites of enzymes, while their heightened reactivity is harnessed for catalysis. However, the latter property renders transition metals toxic at high concentrations. Bacteria, like all living organisms, must regulate the levels of these elements to satisfy their physiological needs while avoiding harm. It is therefore not surprising that the host capitalizes on both the essentiality and toxicity of transition metals to defend against bacterial invaders. This review will discuss established and emerging paradigms in nutrient metal homeostasis at the pathogen-host interface. PMID:22796883

Hood, M. Indriati; Skaar, Eric P.

2013-01-01

100

Critical behavior in the hydrogen insulator-metal transition  

NASA Technical Reports Server (NTRS)

The vibrational Raman spectrum of solid hydrogen has been measured from 77 to 295 K in the vicinity of the recently observed insulator-metal transition and low-temperature phase transition at 150 gigapascals. The measurements provide evidence for a critical point in the pressure-temperature phase boundary of the low-temperature transition. The result suggests that below the critical temperature the insulator-metal transition changes from continuous to discontinuous, consistent with the general criteria originally proposed by Mott (1949) for metallization by band-gap closure. The effect of temperature on hydrogen metallization closely resembles that of the lower-pressure insulator-metal transitions in doped V2O3 alloys.

Hemley, R. J.; Mao, H. K.

1990-01-01

101

Steam Reforming on Transition-metal Carbides from Density-functional Theory  

SciTech Connect

A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.

Vojvodic, Aleksandra

2012-05-11

102

Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis  

USGS Publications Warehouse

Transition metals in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gas generation (Mango, F. D. (1996) Transition metal catalysis in the generation of natural gas. Org. Geochem.24, 977–984). This controversial hypothesis may have important implications concerning gas generation in unconventional shale-gas accumulations. Although experiments have been conducted to test the metal-catalysis hypothesis, their approach and results remain equivocal in evaluating natural assemblages of transition metals and organic matter in shale. The Permian Kupferschiefer of Poland offers an excellent opportunity to test the hypothesis with immature to marginally mature shale rich in both transition metals and organic matter. Twelve subsurface samples containing similar Type-II kerogen with different amounts and types of transition metals were subjected to hydrous pyrolysis at 330° and 355 °C for 72 h. The gases generated in these experiments were quantitatively collected and analyzed for molecular composition and stable isotopes. Expelled immiscible oils, reacted waters, and spent rock were also quantitatively collected. The results show that transition metals have no effect on methane yields or enrichment. ?13C values of generated methane, ethane, propane and butanes show no systematic changes with increasing transition metals. The potential for transition metals to enhance gas generation and oil cracking was examined by looking at the ratio of the generated hydrocarbon gases to generated expelled immiscible oil (i.e., GOR), which showed no systematic change with increasing transition metals. Assuming maximum yields at 355 °C for 72 h and first-order reaction rates, pseudo-rate constants for methane generation at 330 °C were calculated. These rate constants showed no increase with increasing transition metals. The lack of a significant catalytic effect of transition metals on the extent, composition, and timing of natural gas generation in these experiments is attributed to the metals not occurring in the proper form or the poisoning of potential catalytic microcosms by polar-rich bitumen, which impregnates the rock matrix during the early stages of petroleum formation.

Lewan, M.D.; Kotarba, M.J.; Wieclaw, D.; Piestrzynski, A.

2008-01-01

103

Metal-nonmetal transition in the copper-carbon nanocomposite films  

NASA Astrophysics Data System (ADS)

We prepared Cu nanoparticles in a-C:H thin films by co-deposition of RF-sputtering and RF-PECVD methods at room temperature. By increasing Cu content in these films a nonmetal-metal (N-M) transition is observed. This transition is explainable by the power law of percolation theory. The critical metal content is obtained 56% and the critical exponent is obtained 1.6, which is larger than the exponent for 2 dimension systems and smaller than the one for 3 dimension systems. The electrical conductivity of dielectric samples was explained by tunneling. Activation tunneling energy that was obtained from temperature dependence of electrical resistivity correlates with near infrared absorption peak of samples and both of them depend on Cu content of thin films. In the early stage of N-M transition, by increasing metal content, a peculiar effect of metallic to nonmetallic state occurs.

Ghodselahi, T.; Vesaghi, M. A.; Shafiekhani, A.; Ahmadi, M.; Panahandeh, M.; Heidari Saani, M.

2010-09-01

104

Reentrant Metal-Insulator Transitions in Silicon -  

NASA Astrophysics Data System (ADS)

This thesis describes a study of reentrant metal -insulator transitions observed in the inversion layer of extremely high mobility Si-MOSFETs. Magneto-transport measurements were carried out in the temperature range 20mK-4.2 K in a ^3He/^4 He dilution refrigerator which was surrounded by a 15 Tesla superconducting magnet. Below a melting temperature (T_{M}~500 mK) and a critical electron density (n_{s }~9times10^{10} cm^{-2}), the Shubnikov -de Haas oscillations in the diagonal resistivity enormous maximum values at the half filled Landau levels while maintaining deep minima corresponding to the quantum Hall effect at filled Landau levels. At even lower electron densities the insulating regions began to spread and eventually a metal-insulator transition could be induced at zero magnetic field. The measurement of extremely large resistances in the milliKelvin temperature range required the use of very low currents (typically in the 10^ {-12} A range) and in certain measurements minimizing the noise was also a consideration. The improvements achieved in these areas through the use of shielding, optical decouplers and battery operated instruments are described. The transport signatures of the insulating state are considered in terms of two basic mechanisms: single particle localization with transport by variable range hopping and the formation of a collective state such as a pinned Wigner crystal or electron solid with transport through the motion of bound dislocation pairs. The experimental data is best described by the latter model. Thus the two dimensional electron system in these high mobility Si-MOSFETs provides the first and only experimental demonstration to date of the formation of an electron solid at zero and low magnetic fields in the quantum limit where the Coulomb interaction energy dominates over the zero point oscillation energy. The role of disorder in favouring either single particle localization or the formation of a Wigner crystal is explored by considering a variety of samples with a wide range of mobilities and by varying the ratio of the carrier density (controlled by the applied gate voltage) to the impurity density (fixed during sample growth). A phase diagram showing the boundaries between the two dimensional electron gas, the Wigner solid, and the single particle localization induced insulator is established in terms of carrier density and sample mobility.

Campbell, John William M.

105

Band Structure of Transition Metals Studied by ESCA  

Microsoft Academic Search

The position and shape of the energy bands of the following transition metals have been studied by ESCA: Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au. The Fermi levels of the metals with unfilled d-bands are found in the high-energy flanks of the valence band spectra. For the noble metals the Fermi level is shifted toward

Y. Baer; P. F. Hedén; J. Hedman; M. Klasson; C. Nordling; K. Siegbahn

1970-01-01

106

Structural studies on transition metal compounds  

NASA Astrophysics Data System (ADS)

Accurate determination of the electron-density distribution in transition metal complexes provides important information on the electronic ground state of the d electrons placed in a ligand field. The high-spin and low-spin states can be distinguished unequivocally. The degree of hybridization of the component orbitals can also be determined. If the intensity data are good enough, the wavefunction analysis can be carried out and the orbital coefficients can be obtained by a least squares method. In spite of a very small valence/total electron ratio the asphericity of 4d and 5d electrons in a ligand field can be detected. This is owing to sharp deformation density located outside of the high-density inner core region. The crystal structures of a series of dimeric copper(II) carboxylate adducts of the general formula [Cu(RCOO)2L]2 have been accurately determined or redetermined. The temperature-dependent magnetic susceptibility of these crystals indicates that the isolated pairs of Cu(II) ions interact strongly through exchange forces. The present structure investigation has established that there exist no magneto-structural correlations. Instead, it was found that the values of singlet-triplet separation increase roughly linearly with the number of electrons in the bridging OCO group calculated by the molecular orbital method. The chiral cyanoethyl group in a series of cobaloxime complexes was found to racemize on exposure to X-rays without degradation of crystallinity. The rate of racemization was so slow that several structures of intermediate stages could be determined by X-ray analysis. In this way various reaction pathways were recognized and the reaction rate could be correlated quantitatively with the atomic arrangement in the crystal. The related complexes were found to undergo reversible phase transition and/or photoisomerization without degradation of crystallinity.

Saito, Yoshihiko

107

Early Intervention, Positive Behavior Support, and Transition to School  

Microsoft Academic Search

Problem behaviors are addressed most effectively when children are young, through the avenues of family-centered early intervention services.These types of services have a great potential for reducing children's problem behaviors and increasing families' competencies and abilities to resolve future problems. As children age, however, the transitions from early intervention and family-centered services to school-based interventions are associated with a variety

Lise Fox; Glen Dunlap; Lisa Cushing

2002-01-01

108

Quantum phase transition in a common metal.  

PubMed

The classical theory of solids, based on the quantum mechanics of single electrons moving in periodic potentials, provides an excellent description of substances ranging from semiconducting silicon to superconducting aluminium. Over the last fifteen years, it has become increasingly clear that there are substances for which the conventional approach fails. Among these are certain rare earth compounds and transition metal oxides, including high-temperature superconductors. A common feature of these materials is complexity, in the sense that they have relatively large unit cells containing heterogeneous mixtures of atoms. Although many explanations have been put forward for their anomalous properties, it is still possible that the classical theory might suffice. Here we show that a very common chromium alloy has some of the same peculiarities as the more exotic materials, including a quantum critical point, a strongly temperature-dependent Hall resistance and evidence for a 'pseudogap'. This implies that complexity is not a prerequisite for unconventional behaviour. Moreover, it should simplify the general task of explaining anomalous properties because chromium is a relatively simple system in which to work out in quantitative detail the consequences of the conventional theory of solids. PMID:12368850

Yeh, A; Soh, Yeong-Ah; Brooke, J; Aeppli, G; Rosenbaum, T F; Hayden, S M

2002-10-01

109

Transition metal-free olefin polymerization catalyst  

DOEpatents

Ethylene and/or propylene are polymerized to form high molecular weight, linear polymers by contacting ethylene and/or propylene monomer, in the presence of an inert reaction medium, with a catalyst system which consists essentially of (1) an aluminum alkyl component, such as trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-octylaluminum and diethylaluminum hydride and (2) a Lewis acid or Lewis acid derivative component, such as B (C.sub.6 F.sub.5).sub.3, [(CH.sub.3).sub.2 N (H) (C.sub.6 H.sub.5)].sup.+ [B (C.sub.6 F.sub.5)4].sup.-, [(C.sub.2 H.sub.5).sub.3 NH].sup.+ [B C.sub.6 F.sub.5).sub.4 ],.sup.-, [C(C.sub.6 F.sub.5).sub.3 ].sup.+ [B(C.sub.6 F.sub.5).sub.4 ].sup.-, (C.sub.2 H.sub.5).sub.2 Al(OCH.sub.3), (C.sub.2 H.sub.5).sub.2 Al(2,6-di-t-butyl-4-methylphenoxide), (C.sub.2 H.sub.5)Al(2,6 -di-t-butylphenoxide).sub.2, (C.sub.2 H.sub.5).sub.2 Al(2,6-di-t-butylphonoxide) , 2,6 -di-t-butylphenol.multidot.methylaluminoxane or an alkylaluminoxane, and which may be completely free any transition metal component(s).

Sen, Ayusman (State College, PA); Wojcinski, II, Louis M. (State College, PA); Liu, Shengsheng (State College, PA)

2001-01-01

110

Electronic entanglement in late transition metal oxides  

E-print Network

Here we present a study of the entanglement in the electronic structure of the late transition metal monoxides - MnO, FeO, CoO, and NiO - obtained by means of density-functional theory in the local density approximation combined with dynamical mean-field theory (LDA+DMFT). The impurity problem is solved through Exact Diagonalization (ED), which grants full access to the thermally mixed many-body ground state density operator. The quality of the electronic structure is affirmed through a direct comparison between the calculated electronic excitation spectrum and photoemission experiments. Our treatment allows for a quantitative investigation of the entanglement in the electronic structure. Two main sources of entanglement are explicitly resolved through the use of a fidelity based geometrical entanglement measure, and additional information is gained from a complementary entropic entanglement measure. We show that the interplay of crystal field effects and Coulomb interaction causes the entanglement in CoO to take a particularly intricate form.

Patrik Thunström; Igor Di Marco; Olle Eriksson

2013-01-28

111

Fluctuation-driven electroweak phase transition. [in early universe  

NASA Technical Reports Server (NTRS)

We examine the dynamics of the electroweak phase transition in the early Universe. For Higgs masses in the range 46 less than or = M sub H less than or = 150 GeV and top quark masses less than 200 GeV, regions of symmetric and asymmetric vacuum coexist to below the critical temperature, with thermal equilibrium between the two phases maintained by fluctuations of both phases. We propose that the transition to the asymmetric vacuum is completed by percolation of these subcritical fluctuations. Our results are relevant to scenarios of baryogenesis that invoke a weakly first-order phase transition at the electroweak scale.

Gleiser, Marcelo; Kolb, Edward W.

1992-01-01

112

METAL-NON METAL TRANSITIONS /N RARE EARTH COMPOUNDS. EXPERIMENT AND THEORK /.  

E-print Network

METAL-NON METAL TRANSITIONS /N RARE EARTH COMPOUNDS. EXPERIMENT AND THEORK /. VALENCE INSTABILITIES, superconductivity, electron-phonon and band theory, to name a few. 2. Properties of normal rare earth metals. - Before discussing rare earth valence instabilities, three relevant general features of rare earth metals

Paris-Sud XI, Université de

113

Cage complexes of transition metals in biochemistry and medicine  

Microsoft Academic Search

The review presents the following main trends and the perspectives of application of the transition metal clathrochelates\\u000a in medicine and biochemistry: encapsulation of radioactive metal ion for diagnostics and therapy; imaging agents for MRI;\\u000a antidotes and prolonged pharmaceuticals; pharmaceuticals for boron neutron capture therapy; antihelminthic and antiparasitic\\u000a detergents; antioxidants; membrane transport of the metal ions; interaction of the cage metal

Ya. Z. Voloshin; O. A. Varzatskii; Yu. N. Bubnov

2007-01-01

114

Early Incarceration Spells and the Transition to Adulthood  

E-print Network

Early Incarceration Spells and the Transition to Adulthood Steven Raphael Goldman School of Public or prison time (measured as having been interviewed for the survey while incarcerated) and four conventional-post incarceration changes in outcomes for those who are incarcerated relative to similar life-course changes

Sekhon, Jasjeet S.

115

Early Warning Signals of Ecological Transitions: Methods for Spatial Patterns  

PubMed Central

A number of ecosystems can exhibit abrupt shifts between alternative stable states. Because of their important ecological and economic consequences, recent research has focused on devising early warning signals for anticipating such abrupt ecological transitions. In particular, theoretical studies show that changes in spatial characteristics of the system could provide early warnings of approaching transitions. However, the empirical validation of these indicators lag behind their theoretical developments. Here, we summarize a range of currently available spatial early warning signals, suggest potential null models to interpret their trends, and apply them to three simulated spatial data sets of systems undergoing an abrupt transition. In addition to providing a step-by-step methodology for applying these signals to spatial data sets, we propose a statistical toolbox that may be used to help detect approaching transitions in a wide range of spatial data. We hope that our methodology together with the computer codes will stimulate the application and testing of spatial early warning signals on real spatial data. PMID:24658137

Brock, William A.; Carpenter, Stephen R.; Ellison, Aaron M.; Livina, Valerie N.; Seekell, David A.; Scheffer, Marten; van Nes, Egbert H.; Dakos, Vasilis

2014-01-01

116

Mechanisms, mechanics and function of epithelialmesenchymal transitions in early development  

E-print Network

or are not obvious in culture systems or cancer cells. Developmental EMTs are part of a specific differentiative pathMechanisms, mechanics and function of epithelial­mesenchymal transitions in early development David a morphogenic function in shaping the embryo and a patterning function in bringing about new juxtapositions

Keller, Ray

117

Be Prepared: Tips for Transitioning into Early Childhood Education  

ERIC Educational Resources Information Center

The first days of school can be scary for parents, too. As a child reaches an age where she is ready to begin school, the very thought can overwhelm any parent. As a parent of a child with a disability there are added considerations. Nonetheless, transitions through the early years of school can become relatively easier for both the parents and…

Oliver, Laura Ann

2008-01-01

118

I. Experimental test of "carbon monoxide self-shielding" in the early solar nebula using ultra-high resolution VUV photodissociation & photoionization. II. Laser ablation, photodissociation, and photoionization of transition-metal compounds  

NASA Astrophysics Data System (ADS)

Vacuum ultraviolet (VUV) laser spectroscopy, laser ablation, photoion and photoelectron imaging are powerful physical chemistry methods used to study the kinetics of molecules and the dynamics of reactions. This fundamental information is required by astrochemists to test cosmochemical models, and to properly interpret and separate intermingled spectra of molecules in space. The first half of this dissertation discusses our experimental results from testing the "carbon monoxide self-shielding" (COSS) model using laser spectroscopy. Our results reproduced the mass-independent oxygen anomaly observed in the solar system, which confirmed the validity of the model. Additionally, we discuss the results of our photoion imaging study on CO. This study evaluated the chemical pathways of oxygen atoms that are generated from the self-shielded photodissocaiton of CO. The second half of the dissertation focuses on our photoionization studies and theoretical studies on nickel atoms and nickel carbide. This study is the first to accurately measure the ionization energy of NiC. The resulting high-resolution spectra revealed previously unknown electronic transitions of Ni and NiC, which can be used to interpret space spectra, and to benchmark theoretical computations of transition-metal-bearing compounds.

Shi, Xiaoyu

119

Process for making transition metal nitride whiskers  

DOEpatents

A process for making metal nitrides, particularly titanium nitride whiskers, using a cyanide salt as a reducing agent for a metal compound in the presence of an alkali metal oxide. Sodium cyanide, various titanates and titanium oxide mixed with sodium oxide react to provide titanium nitride whiskers that can be used as reinforcement to ceramic composites.

Bamberger, Carlos E. (Oak Ridge, TN)

1989-01-01

120

Phase Transitions in the Early Universe (Is there a Strongly First Order Electroweak Phase Transition?)  

E-print Network

After some introductory remarks about the prospects of first order phase transitions in the early universe, we discuss in some detail the electroweak phase transition. In the standard model case a clear picture is arising including perturbative and nonperturbative effects. Since in this case the phase transition is not strongly first order as needed for baryogenesis, we discuss supersymmetric variants of the standard model, the MSSM with a light stop$_R$ and a NMSSM model where this can be achieved. We conclude with some remarks about the technical procedure and about possible effects of a strongly first order electroweak phase transition including baryogenesis.

M. G. Schmidt

1998-11-30

121

The interactions between transition metal nanoparticles and their metal-oxide supports are often critical for heterogeneous metal nanoparticle  

E-print Network

are often critical for heterogeneous metal nanoparticle catalysts (1). However, the roles of the speciesReports The interactions between transition metal nanoparticles and their metal- oxide supports at the metal-support interface is widely regarded as the key step in room-temperature CO oxidation (13

Napp, Nils

122

Radiative properties of molybdenum sulfide and other transition metal dichalcogenides .  

E-print Network

??Low temperature radiative properties of the layered transition metal dichalcogenides 2H-MoS2, 2H-WS2 and 2H-WSe 2 are investigated. Synthetically grown crystals of all three materials, natural… (more)

Charron, Luc G

2013-01-01

123

Electrolytic separation of crystals of transition-metal oxides  

NASA Technical Reports Server (NTRS)

Versatile flux system grows large, well-formed, stoichiometric single crystals of mixed oxides of the transition-metal elements. These crystals have important uses in the microwave field, and applications as lasers and masers in communications.

Arnott, R. J.; Feretti, A.; Kunnamann, W.

1969-01-01

124

Photochemical reductive elimination of halogen from transition metal complexes  

E-print Network

This thesis is focused on the synthesis and study of transition metal complexes that undergo halogen elimination when irradiated with UV and visible light. This chemistry is relevant for solar energy storage schemes in ...

Cook, Timothy R. (Timothy Raymond), 1982-

2010-01-01

125

Optical properties of two-dimensional transition metal dichalcogenides  

E-print Network

The re-discovery of the atomically thin transition metal dichalcogenides (TMDs), which are mostly semiconductors with a wide range of band gaps, has diversified the family of two-dimensional materials and boosted the ...

Lin, Yuxuan, S.M. Massachusetts Institute of Technology

2014-01-01

126

Charge, orbital and magnetic ordering in transition metal oxides   

E-print Network

Neutron and x-ray diffraction has been used to study charge, orbital and magnetic ordering in some transition metal oxides. The long standing controversy regarding the nature of the ground state (Verwey structure) of the ...

Senn, Mark Stephen

2013-06-29

127

Nitrene transfer reactions by late transition metal complexes  

E-print Network

This thesis presents nitrene transfer reactions that are catalyzed or mediated by late transition metal complexes. Sterically large, fluorinated supporting ligands are used to minimize potential side reactions. A new ...

Hamilton, Charles W., Ph. D. Massachusetts Institute of Technology

2007-01-01

128

Ambipolar Field Effect Transistors Based on Transition-Metal Dichalcogenides  

Microsoft Academic Search

A novel class of the field-effect transistors (FETs) based on transition-metal dichalcogenides has been fabricated. In the single crystals of these layered semiconductors, a weak Van der Waals inter-layer bonding is combined with a strong covalent\\/ionic intra-layer bonding. These materials represent a class of layered inorganic semiconductors with a chemical formula MX_2, where M stands for a transition metal and

Vitaly Podzorov; Michael E. Gershenson; Christian Kloc; Roswitha Zeis; Ernst Bucher

2004-01-01

129

Microwave absorption of mixed-layer transition metal dichalcogenides  

Microsoft Academic Search

‘Mixed-layer mechanism’, a new concept in microwave absorption, was observed in mixed-layer transition-metal dichalcogenides. Two things were observed: first, the strong microwave absorption in the mixed-layer transition metal dichalcogenides, MXM?1 ? Xch2 (M = Re, Ru and Os; M? = Nb and Ta; ch = S and Se); second, the low microwave absorption in the nominally-perfect stacking 3R-MoS2 and MXM?1

K. Hayashi; D. Serikawa; Y. Chijimatsu; M. Shimakawa; S. Kume; K. Manabe; T. Takahashi

1997-01-01

130

The Electronic Structure of Transition Metal Coated Fullerenes  

Microsoft Academic Search

Clusters composed of fullerene molecules with an outer shell of transition metal atoms in the composition C_60M_62 (M being a transition metal) have been produced with laser vaporisation techniques(F. Tast, N. Malinowski, S. Frank, M. Heinebrodt, I.M.L. Billas, and T. P. Martin, Z. Phys D 40), 351 (1997).. We have studied several of these very large systems with a parallel

David C. Patton; Mark R. Pederson; Efthimios Kaxiras

1998-01-01

131

Marginal quantum criticality of metal-insulator transitions  

Microsoft Academic Search

Critical exponents of metal-insulator transitions are studied for an extended Hubbard model within the Hartree-Fock approximation. At zero temperature, we find that the first-order transition line between a symmetry broken (antiferromagnetic or charge-ordered) metal and an insulator terminates at the “marginally quantum critical point”, where an unconventional universality class emerges. This universality is in agreement with the scaling theory and

Takahiro Misawa; Youhei Yamaji; Masatoshi Imada

2007-01-01

132

Synthesis of Nanoporous Iminodiacetic Acid Sorbents for Binding Transition Metals  

PubMed Central

Iminodiacetic acid (IDAA) forms strong complexes with a wide variety of metal ions. Using self-assembled monolayers in mesoporous supports (SAMMS) to present the IDAA ligand potentially allows for multiple metal-ligand interactions to enhance the metal binding affinity relative to that of randomly oriented polymer-based supports. This manuscript describes the synthesis of a novel nanostructured sorbent material built using self-assembly of a IDAA ligand inside a nanoporous silica, and demonstrates its use for capturing transition metal cations, and anionic metal complexes, such as PdCl4?2. PMID:22068901

Busche, Brad; Wiacek, Robert; Davidson, Joseph; Koonsiripaiboon, View; Yantasee, Wassana; Addleman, R. Shane; Fryxell, Glen E.

2009-01-01

133

Electron energy loss studies of the transition metal dichalcogenides  

Microsoft Academic Search

AbstractAn analysis of electron energy loss spectra of some 12 transition metal dichalcogenides has given useful information towards the understanding of the band structures of these layer-type solids. Using the experience gained from studying graphite in which the valence band consists of two groups, namely ? and ? bands, a similar band scheme has also been established for the transition

W. Y. Liang; S. L. Cundy

1969-01-01

134

Induced transition metal moments in rare-earth (yttrium) compounds  

NASA Astrophysics Data System (ADS)

The exchange interactions were analysed in rare-earth (yttrium) transition metal compounds. A transition from nonmagnetic to magnetic state was shown for Co and Ni at ? 70 T and ? 35 T, respectively. The temperature dependences of the magnetic susceptibilities, for the exchange enhanced paramagnets, which change from a T2 dependence to a Curie-Weiss behaviour, as temperature increases, is also analysed.

Burzo, E.

2007-04-01

135

[Non-empirical interatomic potentials for transition metals  

SciTech Connect

The report is divided into the following sections: potential-energy functions for d-band metals, potential-energy functions for aluminides and quasicrystals, electronic structure of complex structures and quasicrystals, potential-energy functions in transition-metal oxides, applications to defect structure and mechanical properties, and basic theory of interatomic potentials.

Not Available

1993-01-01

136

[Non-empirical interatomic potentials for transition metals]. Progress report  

SciTech Connect

The report is divided into the following sections: potential-energy functions for d-band metals, potential-energy functions for aluminides and quasicrystals, electronic structure of complex structures and quasicrystals, potential-energy functions in transition-metal oxides, applications to defect structure and mechanical properties, and basic theory of interatomic potentials.

Not Available

1993-05-01

137

Quantum Critical ``Opalescence'' around Metal-Insulator Transitions  

Microsoft Academic Search

Divergent carrier-density fluctuations equivalent to the critical opalescence of gas-liquid transition emerge around a metal-insulator critical point at a finite temperature. In contrast to the gas-liquid transitions, however, the critical temperatures can be lowered to zero, which offers a challenging quantum phase transition. We present a microscopic description of such quantum critical phenomena in two dimensions. The conventional scheme of

Takahiro Misawa; Youhei Yamaji; Masatoshi Imada

2006-01-01

138

Route to transition metal carbide nanoparticles through cyanamide and metal oxides  

SciTech Connect

We have designed an efficient route to the synthesis of transition metal carbide nanoparticles starting from an organic reagent cyanamide and transition metal oxides. Four technologically important metal carbide nanoparticles such as tungsten carbide, niobium carbide, tantalum carbide and vanadium carbide were synthesized successfully at moderate temperatures. It is found that cyanamide is an efficient carburization reagent and that the metal oxides are completely transmitted into the corresponding carbide nanoparticles. A possible mechanism is proposed to explain the results of the reaction between cyanamide and the metal oxides.

Li, P.G. [Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018 (China)], E-mail: peigangiphy@yahoo.com.cn; Lei, M.; Tang, W.H. [Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018 (China)

2008-12-01

139

Ab initio study of the half-metal to metal transition in strained magnetite  

E-print Network

does not lead to a metallic state for magnetite films grown on (111)-oriented substrates. 3 PresentAb initio study of the half-metal to metal transition in strained magnetite Martin Friák1 of magnetite under different strain conditions. The effects of volume relaxation and internal degrees

140

Theory of metal insulator transition in strongly correlated electron systems  

SciTech Connect

A comprehensive theory of the correlation driven metal insulator transition in 1D and 2D strongly correlated electron systems is given. In both the 1D and 2D Hubbard model the metal insulator transition encountered close to half filling is of Pokrovsky-Talapov type. An important consequence of this in the 2D Hubbard model is the break down of the Fermi liquid theory. We also describe in detail the properties of the Pokrovsky-Talapov transition in 2D ferroelectrics.

Gulacsi, M.; Bedell, K.S.

1994-06-01

141

Metal - non-metal transition and the second critical point in expanded metals  

E-print Network

Based on the non-relativistic Coulomb model within which the matter is a system of interacting electrons and nuclei, using the quantum field theory and linear response theory methods, opportunity for the existence of the second critical point in expanded metals, which is directly related to the metal--nonmetal transition, predicted by Landau and Zeldovitch, is theoretically justified. It is shown that the matter at the second critical point is in the state of true dielectric with zero static conductivity. The results obtained are in agreement with recent experiments for expanded metals. The existence of the second critical point is caused by the initial multi-component nature of the matter consisting of electrons and nuclei and the long-range character of the Coulomb interaction. (Accepted in PTEP)

V. B. Bobrov; S. A. Trigger; A. G. Zagorodny

2013-02-16

142

Thermodynamic behavior near a metal-insulator transition  

NASA Technical Reports Server (NTRS)

Measurements of the low-temperature specific heat of phosphorus-doped silicon for densities near the metal-insulator transition show an enhancement over the conduction-band itinerant-electron value. The enhancement increases toward lower temperatures but is less than that found for the spin susceptibility. The data are compared with various theoretical models; the large ratio of the spin susceptibility to specific heat indicates the presence of localized spin excitations in the metallic phase as the metal-insulator transition is approached.

Paalanen, M. A.; Graebner, J. E.; Bhatt, R. N.; Sachdev, S.

1988-01-01

143

Flexible metallic seal for transition duct in turbine system  

DOEpatents

A turbine system is disclosed. In one embodiment, the turbine system includes a transition duct. The transition duct includes an inlet, an outlet, and a passage extending between the inlet and the outlet and defining a longitudinal axis, a radial axis, and a tangential axis. The outlet of the transition duct is offset from the inlet along the longitudinal axis and the tangential axis. The transition duct further includes an interface member for interfacing with a turbine section. The turbine system further includes a flexible metallic seal contacting the interface member to provide a seal between the interface member and the turbine section.

Flanagan, James Scott; LeBegue, Jeffrey Scott; McMahan, Kevin Weston; Dillard, Daniel Jackson; Pentecost, Ronnie Ray

2014-04-22

144

Metallization and charge-transfer gap closure of transition-metal iodides under pressure  

SciTech Connect

It is shown with resistivity and near-IR absorption measurements that NiI{sub 2}, CoI{sub 2}, and FeI{sub 2} metallize under pressure by closure of the charge-transfer energy gap at pressures of 17, 10, and 23 GPa, respectively, which is close to the antiferromagnetic-diamagnetic transition in NiI{sub 2} and CoI{sub 2}. Thus, the magnetic transitions probably are caused by the metallization; in NiI{sub 2} and CoI{sub 2}, the insulator-metal transitions are first order. Moessbauer and XRD data were also collected. Figs, 46 refs.

Chen, A. Li-Chung

1993-05-01

145

High valence 3p and transition metal based MOFs.  

PubMed

This article focuses on high valence 3p and transition metal based metal organic frameworks. In the first part we will discuss the complex solution chemistry of these metals which makes this sub-class of MOFs more of a challenge than the traditional low valence metal based MOFs. This is followed by a short review of the different classes of solids based on phosphonates, carboxylates and other linkers. Finally, we report some of the most relevant properties of these solids such as their chemical or thermal stability as well as their catalytic, redox- and photo-activities. PMID:24947910

Devic, Thomas; Serre, Christian

2014-08-21

146

[Transition metal mediated transformations of small molecules  

SciTech Connect

Work on organotransition metal chemistry, homogeneous and heterogeneous catalysis is summarized. Several cationic palladium(II) complexes with bulky phosphine or pyridine ligands were discovered that are highly selective catalysts for linear dimerization of vinyl monomers and linear polymerization of p-divinylbenzene, the reactions proceeding through a carbocationic mechanism. Our studies were continued on alternating olefin-carbon monoxide copolymers. The copolymerization reaction and reactivity of copolymers were examined. New catalytic systems for alternating copolymerization of [alpha]-olefins with CO were discovered. In the case of styrene derivatives, tactic copolymers were obtained. Poly(ethylenepyrrolediyl) derivatives were synthesized from alternating ethylene-carbon monoxide copolymer and become electronic conductors when doped with iodine. A catalytic system for direct synthesis of polyureas and polyoxamides from and diamines was also discovered. Pt metal catalyzed the oxidation of ethers, esters, and amines to carboxylic acids and the oxidation of olefins to 1,2-diols. Anaerobic and aerobic decomposition of molybdenum(VI)-oxoalkyl compounds were studied for heterogeneous oxidation of alkanes and olefins on Mo(VI)-oxide surfaces. Synthesis of polymer-trapped metal, metal oxide, and metal sulfide nanoclusters (size <1--10 nm) was studied.

Sen, A.

1992-01-01

147

Early warning signals of desertification transitions in semiarid ecosystems  

NASA Astrophysics Data System (ADS)

The identification of early warning signals for regime shifts in ecosystems is of crucial importance given their impact in terms of economic and social effects. We present here the results of a theoretical study on the desertification transition in semiarid ecosystems under external stress. We performed numerical simulations based on a stochastic cellular automaton model, and we studied the dynamics of the vegetation clusters in terms of percolation theory, assumed as an effective tool for analyzing the geometrical properties of the clusters. Focusing on the role played by the strength of external stresses, measured by the mortality rate m , we followed the progressive degradation of the ecosystem for increasing m , identifying different stages: first, the fragmentation transition occurring at relatively low values of m , then the desertification transition at higher mortality rates, and finally the full desertification transition corresponding to the extinction of the vegetation and the almost complete degradation of the soil, attained at the maximum value of m . For each transition we calculated the spanning probabilities as functions of m and the percolation thresholds according to different spanning criteria. The identification of the different thresholds is proposed as an useful tool for monitoring the increasing degradation of real-world finite-size systems. Moreover, we studied the time fluctuations of the sizes of the biggest clusters of vegetated and nonvegetated cells over the entire range of mortality values. The change of sign in the skewness of the size distributions, occurring at the fragmentation threshold for the biggest vegetation cluster and at the desertification threshold for the nonvegetated cluster, offers new early warning signals for desertification. Other new and robust indicators are given by the maxima of the root-mean-square deviation of the distributions, which are attained respectively inside the fragmentation interval, for the vegetated biggest cluster, and inside the desertification interval, for the nonvegetated cluster.

Corrado, Raffaele; Cherubini, Anna Maria; Pennetta, Cecilia

2014-12-01

148

Phase transitions between polytypes and intralayer superstructures in transition metal dichalcogenides  

Microsoft Academic Search

The different polytypic structures observed in transition metal dichalcogenides are described as the result of ordering processes from a common parent disordered polytype structure. Interpolytypic transitions are suggested to be favored by the intrinsic stacking faults induced in the ordering mechanisms. The influence of these mechanisms on the intralayer superstructures is discussed.

H. Katzke; P. Tolédano; W. Depmeier

2004-01-01

149

Quantum Critical “Opalescence” around Metal-Insulator Transitions  

NASA Astrophysics Data System (ADS)

Divergent carrier-density fluctuations equivalent to the critical opalescence of gas-liquid transition emerge around a metal-insulator critical point at a finite temperature. In contrast to the gas-liquid transitions, however, the critical temperatures can be lowered to zero, which offers a challenging quantum phase transition. We present a microscopic description of such quantum critical phenomena in two dimensions. The conventional scheme of phase transitions by Ginzburg, Landau, and Wilson is violated because of its topological nature. It offers a clear insight into the criticalities of metal-insulator transitions (MIT) associated with Mott or charge-order transitions. Fermi degeneracy involving the diverging density fluctuations generates emergent phenomena near the endpoint of the first-order MIT and must shed new light on remarkable phenomena found in correlated metals such as unconventional cuprate superconductors. It indeed accounts for the otherwise puzzling criticality of the Mott transition recently discovered in an organic conductor. We propose to accurately measure enhanced dielectric fluctuations at small wave numbers.

Misawa, Takahiro; Yamaji, Youhei; Imada, Masatoshi

2006-08-01

150

Thermal properties of layered transition-metal dichalcogenides at charge-density-wave transitions  

Microsoft Academic Search

The heat capacity and resistivity at the charge-density-wave (CDW) transitions of several layered transition-metal dichalcogenides are presented. The magnitude of the thermal and resistive anomalies in the 2H polytypes indicates only a small fraction of the Fermi surface is involved in the CDW distortion. The transition from the normal to the incommensurate CDW state at T0 is second order, while

J. M. E. Harper; T. H. Geballe; F. J. Disalvo

1977-01-01

151

Nanodisperse transition metal electrodes (NTME) for electrochemical cells  

DOEpatents

Disclosed are transition metal electrodes for electrochemical cells using gel-state and solid-state polymers. The electrodes are suitable for use in primary and secondary cells. The electrodes (either negative electrode or positive electrode) are characterized by uniform dispersion of the transition metal at the nanoscale in the polymer. The transition metal moiety is structurally amorphous, so no capacity fade should occur due to lattice expansion/contraction mechanisms. The small grain size, amorphous structure and homogeneous distribution provide improved charge/discharge cycling performance, and a higher initial discharge rate capability. The cells can be cycled at high current densities, limited only by the electrolyte conductivity. A method of making the electrodes (positive and negative), and their usage in electrochemical cells are disclosed.

Striebel, Kathryn A. (Oakland, CA); Wen, Shi-Jie (Sunnyvale, CA)

2000-01-01

152

Nanodisperse transition metal electrodes (NTME) for electrochemical cells  

SciTech Connect

Disclosed are transition metal electrodes for electrochemical cells using gel-state and solid-state polymers. The electrodes are suitable for use in primary and secondary cells. The electrodes (either negative electrode or positive electrode) are characterized by uniform dispersion of the transition metal at the nanoscale in the polymer. The transition metal moiety is structurally amorphous, so no capacity fade should occur due to lattice expansion/contraction mechanisms. The small grain size, amorphous structure and homogeneous distribution provide improved charge/discharge cycling performance, and a higher initial discharge rate capability. The cells can be cycled at high current densities, limited only by the electrolyte conductivity. A method of making the electrodes (positive and negative), and their usage in electrochemical cells are disclosed.

Striebel, Kathryn A.; Wen, Shi-Jie

1998-12-01

153

Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications.  

PubMed

Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles. PMID:23137676

Sanpo, Noppakun; Berndt, Christopher C; Wen, Cuie; Wang, James

2013-03-01

154

Hybrid uranium-transition-metal oxide cage clusters.  

PubMed

Transition-metal based polyoxometalate clusters have been known for decades, whereas those built from uranyl peroxide polyhedra have more recently emerged as a family of complex clusters. Here we report the synthesis and structures of six nanoscale uranyl peroxide cage clusters that contain either tungstate or molybdate polyhedra as part of the cage, as well as phosphate tetrahedra. These transition-metal-uranium hybrid clusters exhibit unique polyhedral connectivities and topologies that include 6-, 7-, 8-, 10-, and 12-membered rings of uranyl polyhedra and uranyl ions coordinated by bidentate peroxide in both trans and cis configurations. The transition-metal polyhedra appear to stabilize unusual units built of uranyl polyhedra, rather than templating their formation. PMID:25434424

Ling, Jie; Hobbs, Franklin; Prendergast, Steven; Adelani, Pius O; Babo, Jean-Marie; Qiu, Jie; Weng, Zhehui; Burns, Peter C

2014-12-15

155

Memristor using a transition metal nitride insulator  

DOEpatents

Apparatus is disclosed in which at least one resistive switching element is interposed between at least a first and a second conducting electrode element. The resistive switching element comprises a metal oxynitride. A method for making such a resistive switching element is also disclosed.

Stevens, James E; Marinella, Matthew; Lohn, Andrew John

2014-10-28

156

Simple model for the Darwinian transition in early evolution  

E-print Network

It has been hypothesized that in the era just before the last universal common ancestor emerged, life on earth was fundamentally collective. Ancient life forms shared their genetic material freely through massive horizontal gene transfer (HGT). At a certain point, however, life made a transition to the modern era of individuality and vertical descent. Here we present a minimal model for this hypothesized "Darwinian transition." The model suggests that HGT-dominated dynamics may have been intermittently interrupted by selection-driven processes during which genotypes became fitter and decreased their inclination toward HGT. Stochastic switching in the population dynamics with three-point (hypernetwork) interactions may have destabilized the HGT-dominated collective state and led to the emergence of vertical descent and the first well-defined species in early evolution. A nonlinear analysis of a stochastic model dynamics covering key features of evolutionary processes (such as selection, mutation, drift and HGT...

Arnoldt, Hinrich; Timme, Marc

2015-01-01

157

Transition-metal acceptor complexes in zinc oxide  

NASA Astrophysics Data System (ADS)

The incorporation of hydrogen shallow donors gives rise to Mn2+ fine and hyperfine lines in the electron paramagnetic resonance of ZnO. This cannot be explained by recharging of isolated Mn atoms. First-principles density functional calculations reveal that transition metals readily form complexes with acceptors by a charge-transfer process or by bond formation. This mechanism has implications on both complex partners. Acceptors are neutralized and uncommon charge states of the transition metals are stabilized. Implications for dilute magnetic ZnO are discussed.

Gluba, M. A.; Nickel, N. H.

2013-02-01

158

On superconductivity of high-spin transition metal compounds  

SciTech Connect

The possibility of Cooper instability in transition metal compounds is established based of the concept of the strong interaction in the same unit cell. The multicomponent scattering amplitude of excitations is calculated. The superconductivity equations are derived for compounds of 3d transition metals. It is shown that in the pole approximation, the superconductivity equations can be reduced to the multicomponent superconductivity equations with preset BCS constants. A method is developed for calculating one-orbital constants and constants with different orbitals as functions of the total spin. The concentration ranges of superconducting ordering are obtained for one-orbital equations.

Zaitsev, R. O., E-mail: Zaitsev_rogdai@mail.ru [Moscow Institute of Physics and Technology (State University) (Russian Federation)

2011-11-15

159

Marginal quantum criticality of metal-insulator transitions  

NASA Astrophysics Data System (ADS)

Critical exponents of metal-insulator transitions are studied for an extended Hubbard model within the Hartree-Fock approximation. At zero temperature, we find that the first-order transition line between a symmetry broken (antiferromagnetic or charge-ordered) metal and an insulator terminates at the " marginally quantum critical point", where an unconventional universality class emerges. This universality is in agreement with the scaling theory and explains the exponents obtained by recent experiments on (V,Cr)2O3 and ?- (ET)2Cu(Cn)2Cl in a unified way as a crossover from the marginally quantum universality and the Ising universality.

Misawa, Takahiro; Yamaji, Youhei; Imada, Masatoshi

2007-03-01

160

Fracture Toughness of Metallic Glasses: Ductile-to-Brittle Transition?  

E-print Network

Quantitative understanding of the fracture toughness of metallic glasses, including the associated ductile-to-brittle transitions, is not yet available. Here we use a simple model of plastic deformation in glasses, coupled to an advanced Eulerian level set formulation for solving complex free boundary problems, to calculate the fracture toughness of metallic glasses as a function of the degree of structural relaxation corresponding to different annealing times near the glass temperature. Our main result indicates the existence of an elasto-plastic crack tip instability for sufficiently relaxed glasses, resulting in a marked drop in the toughness, which we interpret as a ductile-to-brittle transition similar to experimental observations.

Rycroft, Chris H

2012-01-01

161

Synthesis, characterization and photocatalytic performance of transition metal tungstates  

NASA Astrophysics Data System (ADS)

Tungstates of divalent transition metals (M IIWO 4, M = Co II, Ni II, Cu II, Zn II) were synthesized by reaction of transition metal nitrates with sodium tungstate. The precipitates were then calcined at 500 °C. The materials were characterized by means of ICP-AES elemental analysis, UV-Visible spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and surface area analysis. The higher activity for the decolourization of Methylene Blue (MB) and Methyl Orange (MO) of ZnWO 4 compared to that of the other investigated tungstates was correlated with its strong tendency of excitons self-trapping.

Montini, Tiziano; Gombac, Valentina; Hameed, Abdul; Felisari, Laura; Adami, Gianpiero; Fornasiero, Paolo

2010-09-01

162

Magnetization Reversal in Transition Metal Doped ZnO Nanoparticles  

NASA Astrophysics Data System (ADS)

We report magnetic properties of transition metal (TM) doped ZnO nanoparticles and compare the doping effects of different transition metal ions into the ZnO matrix. Stoner-Wohlfarth model has been used to study the switching behavior of magnetic moments by observing energy diagram and hysteresis. Low magnetic anisotropy in Zn1-xMxO nanoparticles is observed where M=Fe, Co, Ni and x is the dopant amount. We have considered chemical precipitation technique for the preparation of Zn1-xMxO samples and have compared the reversal processes of magnetic moments with respect to time in the nanoparticles.

Mondal, Trisha; Tripathi, Ajay; Tiwari, Archana

163

Ternary transition metal phosphides: High-temperature superconductors  

PubMed Central

Two systems of ternary transition metal phosphides with the ordered Fe2P-type hexagonal structure are reported. They have the general formula MRuP and MOsP, in which M can be Ti, Zr, or Hf. An onset of the superconducting transition temperature as high as 13.0 K is reported for the ZrRuP compound. PMID:16592838

Barz, H.; Ku, H. C.; Meisner, G. P.; Fisk, Z.; Matthias, B. T.

1980-01-01

164

Intercalation of Metallocenes in the Layered Transition-Metal Dichalcogenides  

Microsoft Academic Search

Novel intercalation complexes of many of the layered transition-metal dichalcogenides incorporating the metallocenes of cobalt and chromium have been prepared. Evidence is presented indicating a configuration in which the five-membered rings of the metallocene guest are situated perpendicular to the slabs of the dichalcogenide host. In the complexes the metallocenes apparently behave as pseudo-alkali metals, transferring an electron to the

Martin B. Dines

1975-01-01

165

Transition metal-catalyzed enantioselective hydrogenation of enamides and enamines.  

PubMed

Transition metal-catalyzed enantioselective hydrogenation of enamides and enamines is one of the most important methods for the preparation of optically active amines. This review describes the recent developments of highly efficient catalytic asymmetric hydrogenation of enamides, and enamines. It specifically focuses on the substrates because hydrogenation of enamides and enamines is highly dependent on the substrates although the chiral metal catalysts play a significant role. PMID:24595741

Zhou, Qi-Lin; Xie, Jian-Hua

2014-01-01

166

Locally critical quantum phase transitions in strongly correlated metals  

Microsoft Academic Search

When a metal undergoes a continuous quantum phase transition, non-Fermi liquid behavior arises near the critical point. It is standard to assume that all low-energy degrees of freedom induced by quantum criticality are spatially extended, corresponding to long-wavelength fluctuations of the order parameter. However, this picture has been contradicted by recent experiments on a prototype system: heavy fermion metals at

Qimiao Si; Silvio Rabello; Kevin Ingersent; Lleweilun Smith

2001-01-01

167

High temperature superconductivity in metallic region near Mott transition  

E-print Network

The spin-singlet superconductivity without phonons is examined in consideration of correlations on an extended Hubbard model. It is shown that the superconductivity requires not only the total correlation should be strong enough but also the density of state around Fermi energy should be large enough, which shows that the high temperature superconductivity could only be found in the metallic region near the Mott metal insulator transition (MIT). Other properties of superconductors are also discussed on these conclusions.

Tian De Cao

2009-06-11

168

Electronic mechanism of hardness enhancement in transition-metal carbonitrides  

NASA Astrophysics Data System (ADS)

Transition-metal carbides and nitrides are hard materials widely used for cutting tools and wear-resistant coatings. Their hardness is not yet understood at a fundamental level. A clue may lie in the puzzling fact that transition-metal carbonitrides that have the rock-salt structure (such as TiCxN1-x) have the greatest hardness for a valence-electron concentration of about 8.4 per cell, which suggests that the hardness may be determined more by the nature of the bonding than by the conventional microstructural features that determine the hardness of structural metals and alloys. To investigate this possibility, we have evaluated the shear modulus of various transition-metal carbides and nitrides using ab initio pseudopotential calculations. Our results show that the behaviour of these materials can be understood on a fundamental level in terms of their electronic band structure. The unusual hardness originates from a particular band of ? bonding states between the non-metal p orbitals and the metal d orbitals that strongly resists shearing strain or shape change. Filling of these states is completed at a valence-electron concentration of about 8.4, and any additional electrons would go into a higher band which is unstable against shear deformations.

Jhi, Seung-Hoon; Ihm, Jisoon; Louie, Steven G.; Cohen, Marvin L.

1999-05-01

169

Critical points of metals of three main groups and selected transition metals  

Microsoft Academic Search

The liquid–gas critical points of metals of the first three main groups and those of selected transition metals have been estimated by scaling with the ionization potential and valence. The critical density is shown to be attached to the percolation threshold of the classically accessible spheres. Corrections to the classically accessible radius are made allowing for deviation from the Coulomb

A. A. Likalter

2002-01-01

170

Metal-insulator transition in epitaxial vanadium sesquioxide thin films  

NASA Astrophysics Data System (ADS)

Of all the transition metal oxides which exhibit metal-insulator transitions (MIT), one of the most extensively studied in recent years is the vanadium sesquioxide (V2O3), both from experimental and theoretical point of view. At a transition temperature of about 160 K at an ambient pressure of 1 atm, pure V2O3 transforms from a rhombohedral paramagnetic metallic (PM) to a monoclinic antiferromagnetic insulating (AFI) phase upon cooling, with a jump in the resistivity of about seven orders of magnitude. Experimental studies have focused more on bulk V2O3 and recently there have been significant interest in thin film fabrication of this material due to potential applications as thermal sensors, current limiters, Positive Temperature Coefficient (PTC) thermistors, and optical switches. This study addresses the deposition, characterization, and properties of high-quality epitaxial V2O3 thin films grown on a-, c-Al2O3 and c-LiTaO 3 substrates by a straightforward method of pulsed laser deposition (PLD). Various characterization techniques including X-ray diffraction, atomic force microscopy, scanning electron microscopy, and X-ray photoemission spectroscopy were used to examine the structural, crystallographic, and surface properties, while four point probe resistivity measurements were used to examine the electrical properties of the films. V2O3 thin films of different thicknesses ranging from 10-450 nm were deposited on c-Al 2O3 and c-LiTaO3 substrates by PLD to understand also the role of epitaxial strains. Resistivity measurements showed that depending on the thicknesses of films, different electrical transitions were exhibited by the samples. While some of the samples displayed the expected metal-insulator transition typical of bulk V2O3, some showed insulating behavior only and others exhibited metallic characteristics only over the whole temperature range. For example, for films on c-LiTaO3 with increasing film thickness, first an insulator-insulator, then a metal-insulator, followed by a metal-metal transition is observed. Thicker films (>202 nm) remain metallic in the temperature range of the measurements.

Allimi, Bamidele S.

171

TOPICAL REVIEW: Structural changes and the metal-non-metal transition in supercritical fluids  

Microsoft Academic Search

Energy-dispersive x-ray diffraction (XD), small-angle x-ray scattering (SAXS) and x-ray absorption fine-structure (XAFS) measurements for metallic fluids up to the supercritical region were carried out using synchrotron radiation. We obtained the structure factor S(k) and the pair distribution functions g(r) for expanded fluid Hg from the liquid to the dense vapour region including the metal-non-metal (M-NM) transition region in the

Kozaburo Tamura; Masanori Inui

2001-01-01

172

The Electronic Structure of Transition Metal Coated Fullerenes  

NASA Astrophysics Data System (ADS)

Clusters composed of fullerene molecules with an outer shell of transition metal atoms in the composition C_60M_62 (M being a transition metal) have been produced with laser vaporisation techniques(F. Tast, N. Malinowski, S. Frank, M. Heinebrodt, I.M.L. Billas, and T. P. Martin, Z. Phys D 40), 351 (1997).. We have studied several of these very large systems with a parallel version of the all-electron NRLMOL cluster code. Optimized geometries of the metal encased fullerenes C_60Ti_62 and C_60V_62 are presented along with their HOMO-LUMO gaps, electron affinities, ionization energies, and cohesive energies. We compare the stability of these clusters to relaxed met-car structures (e.g. Ti_8C_12) and to relaxed rocksalt metal-carbide fragments (TiC)n with n=8 and 32. In addition to metal-coated fullerenes we consider the possibility of a trilayered structure consisting of a small shell of metal atoms enclosed by a metal coated fullerene. The nature of bonding in these systems is analyzed by studying the electronic charge distributions.

Patton, David C.; Pederson, Mark R.; Kaxiras, Efthimios

1998-03-01

173

Metal-insulator transitions due to self-doping  

SciTech Connect

We investigate the influence of an unoccupied band on the transport properties of a strongly correlated electron system. For that purpose, additional orbitals are coupled to a Hubbard model via hybridization. The filling is one electron per site. Depending on the position of the additional band, both a metal-to-insulator and an insulator-to-metal transition occur with increasing hybridization. The latter transition from a Mott insulator into a metal via {open_quote}{open_quote}self-doping{close_quote}{close_quote} was recently proposed to explain the low carrier concentration in Yb{sub 4}As{sub 3}. We suggest a restrictive parameter regime for this transition, making use of exact results in various limits. The predicted absence of the self-doping transition for nested Fermi surfaces is confirmed by means of an unrestricted Hartree-Fock approximation and an exact diagonalization study in one dimension. In the general case metal-insulator phase diagrams are obtained within the slave-boson mean-field and the alloy-analog approximations. {copyright} {ital 1996 The American Physical Society.}

Blawid, S.; Tuan, H.A.; Yanagisawa, T.; Fulde, P. [Max-Planck-Institut fuer Physik komplexer Systeme, Bayreuther Strasse 40, D-01187 Dresden (Germany)] [Max-Planck-Institut fuer Physik komplexer Systeme, Bayreuther Strasse 40, D-01187 Dresden (Germany)

1996-09-01

174

Synthesis, Superstructure, Vacancy-Ordering of Intercalated Transition Metal Dichalcogenides.  

E-print Network

??In this thesis, the synthesis and crystal structures are discussed of intercalated transition metal dichalcogenides (TMDCs) 6R-CuxTa1+yS2 and 2H-CuxTa1+ySe2, as well as the compound Cu3TaSe4.… (more)

Ali, Sk Imran

2014-01-01

175

Band mapping of in situ Cs intercalated transition metal dichalcogenides  

Microsoft Academic Search

The valence band structure of in situ Cs intercalated layered transition metal dichalcogenides has been studied with photoemission spectroscopy. The intercalation was achieved by room temperature deposition of Cs onto clean sample surfaces in UHV. The changes in the valence band structure upon intercalation were more extensive than predicted by the rigid band model, with the most profound change being

H. E. Brauer; H. I. Starnberg; H. P. Hughes; L. J. Holleboom

1996-01-01

176

Hall effect at a tunable metal-insulator transition  

E-print Network

Using a rotating magnetic field, the Hall effect in three-dimensional amorphous GdxSi1-x has been measured in the critical regime of the metal-insulator transition for a constant total magnetic field. The Hall coefficient R-0 is negative, indicating...

Teizer, Winfried; Hellman, F.; Dynes, RC.

2003-01-01

177

Electromagnetic generation of ultrasonic waves in 3-d transition metals  

Microsoft Academic Search

We investigated experimentally acoustic waves generated by electromagnetic means in the surface of 3-d transition metals. The signal amplitude was greatest when the static magnetic field was normal to the generating surface. The amplitude increased in proportion to the external field only up to 3 kOe in both polycrystalline and single crystals of Ni, while there was a tendency for

M. Hanabusa; T. Kushida; J. C. Murphy

1973-01-01

178

Stable singlet carbenes as mimics for transition metal centers  

PubMed Central

This perspective summarizes recent results, which demonstrate that stable carbenes can activate small molecules (CO, H2, NH3 and P4) and stabilize highly reactive intermediates (main group elements in the zero oxidation state and paramagnetic species). These two tasks were previously exclusive for transition metal complexes. PMID:21743834

Martin, David; Soleilhavoup, Michele

2011-01-01

179

Probing the Local Coordination Environment for Transition Metal  

E-print Network

ABSTRACT It is hypothesized that a highly ordered, relatively defect-free dilute magnetic semiconductor correlates well with observed weakly ferromagnetic behavior of the nanowires. Dilute magnetic semiconductors with the magnetic properties of the nanowires. These transition-metal- doped ZnO nanowires, being single crystalline

Yang, Peidong

180

Rare earth transition metal magnesium compounds—An overview  

NASA Astrophysics Data System (ADS)

Intermetallic rare earth-transition metal-magnesium compounds play an important role as precipitations in modern light weight alloys and as host materials for hydrogen storage applications. Recent results on the crystal chemistry, the chemical bonding peculiarities, physical properties, and hydrogenation behavior of these materials are reviewed.

Rodewald, Ute Ch.; Chevalier, Bernard; Pöttgen, Rainer

2007-05-01

181

Building a FET with 2D Transition Metal Dichalcogenides  

E-print Network

transfer method ­ New method using polycarbonate (PC) spun on PDMS, then pick up flakes · Once device ­ Spinning ­ Exfoliating ­ Searching ­ Transferring · Future goals · Conclusion #12;Transition Metal to valleytronics Pelant. Nanocrystalline Silicon for Nanophotonics. file=4

Washington at Seattle, University of - Department of Physics, Electroweak Interaction Research Group

182

Transitions in critical size for metal (100) homoepitaxy  

Microsoft Academic Search

For submonolayer nucleation and growth of islands during metal (100) homoepitaxy, we precisely characterize transitions, with increasing temperature, in the critical size, i, above which islands are effectively stable. For strong adatom bonding, we demonstrate universal crossover behavior from i = 1 directly to a well-defined “i = 3” regime where tetramers are stable, but not, e.g., pentamers. A criterion

M. C. Bartelt; L. S. Perkins; J. W. Evans

1995-01-01

183

Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives.  

PubMed

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems. PMID:24754702

Eksik, Osman; Gao, Jian; Shojaee, S Ali; Thomas, Abhay; Chow, Philippe; Bartolucci, Stephen F; Lucca, Don A; Koratkar, Nikhil

2014-05-27

184

Family structure and the transition to early parenthood.  

PubMed

With the rise in out-of-wedlock childbearing and divorce in the last quarter of the twentieth century, an increasing proportion of children have been exposed to a variety of new family forms. Little research has focused on the consequences of childhood family structure for men's transition to fatherhood or on the family processes that account for the effects of family structure on the likelihood that young women and men become first-time unmarried parents, what we now call "fragile families." The data come from the linked Children and Young Adult samples of the 1979 National Longitudinal Survey of Youth (NLSY79), which provide information on the children of the women of the NLSY79 from birth until they enter young adulthood. Females growing up with a single parent and males experiencing an unstable family transition to parenthood early, particularly to nonresidential fatherhood for males. For males, the effects are strongly mediated by parenting processes and adolescent behaviors and are shaped by economic circumstances. Having experienced multiple transitions as a child is associated with a reduced likelihood that males father their first child within marriage and an increased likelihood that they become fathers within cohabitation, demonstrating how changes in family structure alter family structure patterns over time and generations. PMID:20608104

Hofferth, Sandra L; Goldscheider, Frances

2010-05-01

185

Family Structure and the Transition to Early Parenthood  

PubMed Central

With the rise in out-of-wedlock childbearing and divorce in the last quarter of the twentieth century, an increasing proportion of children have been exposed to a variety of new family forms. Little research has focused on the consequences of childhood family structure for men’s transition to fatherhood or on the family processes that account for the effects of family structure on the likelihood that young women and men become first-time unmarried parents, what we now call “fragile families.” The data come from the linked Children and Young Adult samples of the 1979 National Longitudinal Survey of Youth (NLSY79), which provide information on the children of the women of the NLSY79 from birth until they enter young adulthood. Females growing up with a single parent and males experiencing an unstable family transition to parenthood early, particularly to nonresidential fatherhood for males. For males, the effects are strongly mediated by parenting processes and adolescent behaviors and are shaped by economic circumstances. Having experienced multiple transitions as a child is associated with a reduced likelihood that males father their first child within marriage and an increased likelihood that they become fathers within cohabitation, demonstrating how changes in family structure alter family structure patterns over time and generations. PMID:20608104

HOFFERTH, SANDRA L.; GOLDSCHEIDER, FRANCES

2010-01-01

186

Pseudogap and metal-insulator transitions in doped semiconductors  

SciTech Connect

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, T{sub PG} and T{sub {lambda}}(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, T{sub PG}. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature T{sub {lambda}}(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T{sup *}(x) {approx} T{sub {lambda}}(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La{sub 2-x}Sr{sub x}CUO{sub 4}.

Agafonov, A. I., E-mail: aai@isssph.kiae.ru; Manykin, E. A. [Russian Research Centre Kurchatov Institute (Russian Federation)

2006-01-15

187

Mechanical failure and glass transition in metallic glasses  

SciTech Connect

The current majority view on the phenomenon of mechanical failure in metallic glasses appears to be that it is caused by the activity of some structural defects, such as free-volumes or shear transformation zones, and the concentration of such defects is small, only of the order of 1%. However, the recent results compel us to revise this view. Through molecular dynamics simulation it has been shown that mechanical failure is the stress-induced glass transition. According to our theory the concentration of the liquid-like sites (defects) is well over 20% at the glass transition. We suggest that the defect concentration in metallic glasses is actually very high, and percolation of such defects causes atomic avalanche and mechanical failure. In this article we discuss the glass transition, mechanical failure and viscosity from such a point of view.

Egami, Takeshi [ORNL

2011-01-01

188

Theoretical studies of structural phase transitions in actinide metals  

SciTech Connect

Accurate full-potential, relativistic, total energy electronic structure calculations have been carried out over a range of volumes for selected rare-earth and actinide elements in crystal structures experimentally observed in these elements. Correct zero temperature crystal structures are obtained, and calculated equilibrium properties are in reasonable agreement with experiment. Results of these calculations suggest that the unusual equilibrium crystal structures occurring in the light actinides are characteristic of narrow band metals in general and that the light actinides will exhibit structural phase transitions to high symmetry structures under compression. A balance between one-electron bandwidths and band fillings and the electrostatic energy of the crystal lattice produces a tendency toward high-symmetry structures at broad bandwidths and low-symmetry structures at narrow bandwidths. Allowing for expansion as well as contraction, simple metals, transition metals and actinides can be ``stabilized`` in low- and high-symmetry crystal structures.

Wills, J.M.; Boring, A.M. [Los Alamos National Lab., NM (United States); Eriksson, O.; Soederlind, P. [Uppsala Univ. (Sweden). Dept. of Physics

1993-10-01

189

Quantifying covalency and metallicity in correlated compounds undergoing metal-insulator transitions  

NASA Astrophysics Data System (ADS)

The tunability of bonding character in transition-metal compounds controls phase transitions and their fascinating properties such as high-temperature superconductivity, colossal magnetoresistance, spin-charge ordering, etc. However, separating out and quantifying the roles of covalency and metallicity derived from the same set of transition-metal d and ligand p electrons remains a fundamental challenge. In this study, we use bulk-sensitive photoelectron spectroscopy and configuration-interaction calculations for quantifying the covalency and metallicity in correlated compounds. The method is applied to study the first-order temperature- (T-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates Tl2Ru2O7 and Hg2Ru2O7. Core-level spectroscopy shows drastic T-dependent modifications which are well explained by including ligand-screening and metallic-screening channels. The core-level metallic-origin features get quenched upon gap formation in valence band spectra, while ionic and covalent components remain intact across the MIT. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveal three energy scales: (a) 4d electronic changes occur on the largest (˜eV) energy scale, (b) the band-gap energies/charge gaps (Eg˜160-200 meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature TMIT (˜10 meV), which is also the spin gap energy of Tl2Ru2O7 and the magnetic-ordering temperature of Hg2Ru2O7. The method is general for doping- and T-induced transitions and is valid for V2O3, CrN, La1-xSrxMnO3, La2-xSrxCuO4, etc. The obtained transition-metal-ligand (d-p) bonding energies (V˜45-90 kcal/mol) are consistent with thermochemical data, and with energies of typical heteronuclear covalent bonds such as C-H, C-O, C-N, etc. In contrast, the metallic-screening energies of correlated compounds form a weaker class (V*˜10-40 kcal/mol) but are still stronger than van der Waals and hydrogen bonding. The results identify and quantify the roles of covalency and metallicity in 3d and 4d correlated compounds undergoing metal-insulator transitions.

Chainani, Ashish; Yamamoto, Ayako; Matsunami, Masaharu; Eguchi, Ritsuko; Taguchi, Munetaka; Takata, Yasutaka; Takagi, Hidenori; Shin, Shik; Nishino, Yoshinori; Yabashi, Makina; Tamasaku, Kenji; Ishikawa, Tetsuya

2013-01-01

190

Formation of carbyne and graphyne on transition metal surfaces  

NASA Astrophysics Data System (ADS)

The electronic and geometric structures of carbyne on various transition metal surfaces were investigated by theoretical calculations. It was found that carbyne on non-active metal surfaces has a polyynic structure, while a polycumulenic structure is more stable on active catalyst surfaces. The self-assembly of carbyne on a metal substrate could lead to the synthesis of graphyne.The electronic and geometric structures of carbyne on various transition metal surfaces were investigated by theoretical calculations. It was found that carbyne on non-active metal surfaces has a polyynic structure, while a polycumulenic structure is more stable on active catalyst surfaces. The self-assembly of carbyne on a metal substrate could lead to the synthesis of graphyne. Electronic supplementary information (ESI) available: Computational details and structural information of carbon chains on Ni(111) and Rh(111) surfaces, carbon island on Cu(111), Ni(111), Rh(111) and Ru(0001) surfaces. See DOI: 10.1039/c4nr03757j

Yuan, Qinghong; Ding, Feng

2014-10-01

191

Atomic interactions and phase transition in hydrogen metal systems  

SciTech Connect

The statistical mechanical behavior of H in pure metals and metal alloys is investigated using elasticity theory and lattice-gas models. The interactions between H atoms are separated into elastic and electronic contributions, and only the elastic contributions are evaluated. Three studies are reported: trapping of H by interstitial and substitutional impurities in bbc metals; modeling of H in impure metals; and modeling of H in concentration metal hydrides. The elastic interactions between H and interstitial and substitutional impurities in bbc metals are calculated using the method of lattice statics. Comparison with experimentally-measured trapping energies indicates that trapping by interstitial impurities is due solely to elastic energy, whereas both elastic and electronic interactions contribute to trapping by substitutional impurities. To include the effect of substitutional trapping on the phase transitions of H in metal alloys, the random-field lattice-gas model is developed. Interactions between H atoms are determined using the effective metal atom approximation. Hydrogen-impurity interactions are treated as a random field, in analogy with the random-field Ising model. Phase diagrams of the Nb/sub .95/Mo/sub .05/-H and Nb/sub .85/Mo/sub .15/-H systems are calculated by Monte Carlo simulation using the random-field model. The model gives qualitatively correct predictions of the phase behavior when the random field is included.

Shirley, A.I.

1985-01-01

192

Electronic and bonding analysis of hardness in pyrite-type transition-metal pernitrides  

NASA Astrophysics Data System (ADS)

Most commonly known hard transition-metal nitrides crystallize in rocksalt structure (B1). The discovery of ultraincompressible pyrite-type PtN2 10 years ago has raised a question about the cause of its exceptional mechanical properties. We answer this question by a systematic computational analysis of the pyrite-type PtN2 and other transition-metal pernitrides (MN2) with density functional theory. Apart from PtN2, the three hardest phases are found among them in the 3d transition-metal period. They are MnN2, CoN2, and NiN2, with computed Vickers hardness (HV) values of 19.9 GPa, 16.5 GPa, and 15.7 GPa, respectively. Harder than all of these is PtN2, with a HV of 23.5 GPa. We found the following trends and correlations that explain the origin of hardness in these pernitrides. (a) Charge transfer from M to N controls the length of the N-N bond, resulting in a correlation with bulk modulus, dominantly by providing Coulomb repulsion between the pairing N atoms. (b) Elastic constant C44, an indicator of mechanical stability and hardness is correlated with total density of states at EF, an indicator of metallicity. (c) Often cited monotonic variation of HV and Pugh's ratio with valence electron concentration found in rocksalt-type early transition-metal nitrides is not evident in this structure. (d) The change in M-M bond strength under a shearing strain indicated by crystal orbital Hamilton population is predictive of hardness. This is a direct connection between a specific bond and shear related mechanical properties. This panoptic view involving ionicity, metallicity, and covalency is essential to obtain a clear microscopic understanding of hardness.

Liu, Z. T. Y.; Gall, D.; Khare, S. V.

2014-10-01

193

Syntheses and applications of soluble polyisobutylene (PIB)-supported transition metal catalysts  

E-print Network

Soluble polymer supports facilitate the recovery and recycling of expensive transition metal complexes. Recently, polyisobutylene (PIB) oligomers have been found to be suitable polymer supports for the recovery of a variety of transition metal...

Tian, Jianhua

2009-05-15

194

Strain effects in low-dimensional transition metal oxides Jinbo Cao a,b,  

E-print Network

Strain effects in low-dimensional transition metal oxides Jinbo Cao a,b, *, Junqiao Wu a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 1.1. Spin­lattice­charge coupling in transition metal oxides

Wu, Junqiao

195

Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals  

DOEpatents

Stable cross-linked complexes of transition-metal tetracarboxylates and proteins are formed. The preferred transition-metal is rhodium. The protein may be collagen or an enzyme such as a proteolytic enzyme.

Kostic, Nenad M. (Ames, IA); Chen, Jian (Ames, IA)

1991-03-05

196

An Alternative Approach to the Teaching of Systematic Transition Metal Chemistry.  

ERIC Educational Resources Information Center

Presents an alternative approach to teaching Systematic Transition Metal Chemistry with the transition metal chemistry skeleton features of interest. The "skeleton" is intended as a guide to predicting the chemistry of a selected compound. (Author/SA)

Hathaway, Brian

1979-01-01

197

Probing early-universe phase transitions with CMB spectral distortions  

NASA Astrophysics Data System (ADS)

Global, symmetry-breaking phase transitions in the early universe can generate scaling seed networks which lead to metric perturbations. The acoustic waves in the photon-baryon plasma sourced by these metric perturbations, when Silk damped, generate spectral distortions of the cosmic microwave background (CMB). In this work, the chemical potential distortion (? ) due to scaling seed networks is computed and the accompanying Compton y -type distortion is estimated. The specific model of choice is the O (N ) nonlinear ? -model for N ?1 , but the results remain the same order of magnitude for other scaling seeds. If CMB anisotropy constraints to the O (N ) model are saturated, the resulting chemical potential distortion ? ?2 ×1 0-9 .

Amin, Mustafa A.; Grin, Daniel

2014-10-01

198

Vibrational energy transfer dynamics in ruthenium polypyridine transition metal complexes.  

PubMed

Understanding the dynamics of the initial stages of vibrational energy transfer in transition metal complexes is a challenging fundamental question which is also of crucial importance for many applications, such as improving the performance of solar devices or photocatalysis. The present study investigates vibrational energy transport in the ground and the electronic excited state of Ru(4,4'-(COOEt)2-2,2-bpy)2(NCS)2, a close relative of the efficient "N3" dye used in dye-sensitized solar cells. Using the emerging technique of ultrafast two-dimensional infrared spectroscopy, we show that, similarly to other transition-metal complexes, the central Ru heavy atom acts as a "bottleneck" making the energy transfer from small ligands with high energy vibrational stretching frequencies less favorable and thereby affecting the efficiency of vibrational energy flow in the complex. Comparison of the vibrational relaxation times in the electronic ground and excited state of Ru(4,4'-(COOEt)2-2,2-bpy)2(NCS)2 shows that it is dramatically faster in the latter. We propose to explain this observation by the intramolecular electrostatic interactions between the thiocyanate group and partially oxidised Ru metal center, which increase the degree of vibrational coupling between CN and Ru-N modes in the excited state thus reducing structural and thermodynamic barriers that slow down vibrational relaxation and energy transport in the electronic ground state. As a very similar behavior was earlier observed in another transition-metal complex, Re(4,4'-(COOEt)2-2,2'-bpy)(CO)3Cl, we suggest that this effect in vibrational energy dynamics might be common for transition-metal complexes with heavy central atoms. PMID:25463745

Fedoseeva, Marina; Delor, Milan; Parker, Simon C; Sazanovich, Igor V; Towrie, Michael; Parker, Anthony W; Weinstein, Julia A

2015-01-21

199

Wetting Transition of Classical Gases on Alkali Metals  

NASA Astrophysics Data System (ADS)

This talk will review recent discoveries of wetting transitions of classical gases on alkali metals. The transitions occur because the adsorption potentials on these surfaces are less attractive than those on any other surface(A. Chizmeshya, M. W. Cole, and E. Zaremba, J. Low Temp. Phys., in press.). Hence, relatively little adsorption occurs at temperature (T) close to the triple temperature. At higher T, wetting transitions are expected to occur, according to general arguments of Cahn and Ebner and Saam. Indeed, these have been found for all inert light gases on Cs and Rb. The talk will focus on recent experiments and simulations for the case of Ne, the results of which include evidence of a drying transition(G. B. Hess, M. J. Sabatini and M. H. W. Chan, Phys. Rev. Lett. 78, 1739 (1997); M. J. Bojan, M. W. Cole, J. K. Johnson, W. A. Steele and Q. Wang, J. Low Temp. Phys., in press.). Experiments are proposed to test predictions of wetting transitions for Ar and Kr on alkali metals.

Cole, Milton W.

1998-03-01

200

Thermal properties of zirconium diboride -- transition metal boride solid solutions  

NASA Astrophysics Data System (ADS)

This research focuses on the thermal properties of zirconium diboride (ZrB2) based ceramics. The overall goal was to improve the understanding of how different transition metal (TM) additives influence thermal transport in ZrB2. To achieve this, ZrB2 with 0.5 wt% carbon, and 3 mol% of individual transition metal borides, was densified by hot-press sintering. The transition metals that were investigated were: Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, and Re. The room temperature thermal diffusivities of the compositions ranged from 0.331 cm2/s for nominally pure ZrB2 to 0.105 cm2/s for (Zr,Cr)B2 and converged around 0.155cm2/s at higher temperatures for all compositions. Thermal conductivities were calculated from the diffusivities, using temperature-dependent values for density and heat capacity. The electron contribution to thermal conductivity was calculated from measured electrical resistivity according to the Wiedemann-Franz law. The phonon contribution to thermal conductivity was calculated by subtracting the electron contribution from the total thermal conductivity. Rietveld refinement of x-ray diffraction data was used to determine the lattice parameters of the compositions. The decrease in thermal conductivity for individual additives correlated directly to the metallic radius of the additive. Additional strain appeared to exist for additives when the stable TM boride for that metal had different crystal symmetries than ZrB2. This research provided insight into how additives and impurities affect thermal transport in ZrB2. The research potentially offers a basis for future modeling of thermal conductivity in ultra-high temperature ceramics based on the correlation between metallic radius and the decrease in thermal conductivity.

McClane, Devon Lee

201

Prospective Associations Among Assets and Successful Transition to Early Adulthood.  

PubMed

Objectives. We investigated prospective associations among assets (e.g., family communication), which research has shown to protect youths from risk behavior, and successful transition to early adulthood (STEA). Methods. We included participants (n?=?651) aged 18 years and older at study wave 5 (2007-2008) of the Youth Asset Study, in the Oklahoma City, Oklahoma, metro area, in the analyses. We categorized 14 assets into individual-, family-, or community-level groups. We included asset groups assessed at wave 1 (2003-2004) in linear regression analyses to predict STEA 4 years later at wave 5. Results. Individual- and community-level assets significantly (P?early adulthood. PMID:25393188

Oman, Roy F; Vesely, Sara K; Aspy, Cheryl B; Tolma, Eleni L

2015-01-01

202

Energy transitions in the early 21st Century  

SciTech Connect

We are in the early stages of a long and complex transition from a global economy based on fossil energy to an economy based on low carbon renewable energy. However, fossil fuel resources are abundant and widely distributed, and they will remain the dominant source of primary energy for at least the next quarter century. In the United States, displacement of coal by natural gas for electric power generation has done more to reduce CO2 emissions than all new renewables combined, and this may occur globally for the next decade or two, even if the European Union does not take advantage of its large unconventional natural gas resources. Greater energy efficiency (not including the efficiencies associated with displacement of coal by gas) will also be more important than new renewables. Cost/benefit ratios are important for sustainability of the transition, and some energy efficiency technologies and displacement of coal by natural gas have lower cost/benefit ratios than wind power, solar power or biofuels. Money spent on the large scale deployment of wind, solar and especially biofuels would be better spent on research, development and demonstration of a broader suite of technologies that would support the energy transition, with a focus on improving the cost benefit ratios of already deployed technologies and developing alternatives. Advanced nuclear reactors, engineered geothermal systems, fossil fuel recovery coupled with CO2 sequestration and pre-combustion or post-combustion decarbonation of fossil fuels with geological CO2 sequestration are among the technologies that might be more cost effective than wind, solar or biofuels, and biofuels have serious adverse societal and environment consequences.

Paul Meakin

2013-01-01

203

Holographic Polarons, the Metal-Insulator Transition and Massive Gravity  

E-print Network

Massive gravity is holographically dual to `realistic' materials with momentum relaxation. The dual graviton potential encodes the phonon dynamics and it allows for a much broader diversity than considered so far. We construct a simple family of isotropic and homogeneous materials that exhibit an interaction-driven Metal-Insulator transition. The transition is triggered by the formation of polarons -- phonon-electron quasi-bound states that dominate the conductivities, shifting the spectral weight above a mass gap. We characterize the polaron gap, width and dispersion.

Baggioli, Matteo

2014-01-01

204

Holographic Polarons, the Metal-Insulator Transition and Massive Gravity  

E-print Network

Massive gravity is holographically dual to `realistic' materials with momentum relaxation. The dual graviton potential encodes the phonon dynamics and it allows for a much broader diversity than considered so far. We construct a simple family of isotropic and homogeneous materials that exhibit an interaction-driven Metal-Insulator transition. The transition is triggered by the formation of polarons -- phonon-electron quasi-bound states that dominate the conductivities, shifting the spectral weight above a mass gap. We characterize the polaron gap, width and dispersion.

Matteo Baggioli; Oriol Pujolas

2014-11-19

205

Role of Electronic Structure in the Susceptibility of Metastable Transition-Metal Oxide Structures to Transformation  

E-print Network

Role of Electronic Structure in the Susceptibility of Metastable Transition-Metal Oxide Structures transition-metal ions in an oxide and how these factors in turn affect the resistance of metastable 3d transition-metal oxides against transformation. This is a relevant topic to the Li rechargeable battery field

Ceder, Gerbrand

206

Understanding the NMR shifts in paramagnetic transition metal oxides using density functional theory calculations  

E-print Network

Understanding the NMR shifts in paramagnetic transition metal oxides using density functional­14 however, no such studies, to our knowledge, have been performed in transition metal oxides or other of the lithium local environment and electronic configuration of the transition metal ions. We focus

Ceder, Gerbrand

207

Wigner-Mott Quantum Criticality and the Two-Dimensional Metal-Insulator Transition  

E-print Network

Wigner-Mott Quantum Criticality and the Two-Dimensional Metal-Insulator Transition Vladimir #12;Wigner-Mott Quantum Criticality and the Two-Dimensional Metal-Insulator Transition Vladimir ...and the Sorcerer's Stone The Saga of Metal-Insulator Transitions: From Alchemy to Quantum Magic #12

Dobrosavljevic, Vladimir

208

X-ray Absorption Spectroscopy of Transition Metal-Magnesium Hydride Thin Films  

E-print Network

X-ray Absorption Spectroscopy of Transition Metal-Magnesium Hydride Thin Films T. J. Richardsona@lbl.gov Abstract Mixed metal thin films containing magnesium and a first-row transition element exhibit very large and coordination of the magnesium and transition metal atoms during hydrogen absorption were studied using dynamic

209

Thermochemistry of alloys of transition metals and lanthanide metals with some IIIB and IVB elements in the periodic table  

Microsoft Academic Search

We have conducted systematic studies in this laboratory of the thermochemistry of transition metal and rare earth alloys by high temperature calorimetric methods. An overview of the thermochemistry of the alloys of the transition metals and lanthanide metals with elements in the IIIB and IVB columns of the periodic table is presented. The enthalpies of formation of most of these

S. V. Meschel; O. J. Kleppa

2001-01-01

210

Tailoring magnetic skyrmions in ultra-thin transition metal films.  

PubMed

Skyrmions in magnetic materials offer attractive perspectives for future spintronic applications since they are topologically stabilized spin structures on the nanometre scale, which can be manipulated with electric current densities that are by orders of magnitude lower than those required for moving domain walls. So far, they were restricted to bulk magnets with a particular chiral crystal symmetry greatly limiting the number of available systems and the adjustability of their properties. Recently, it has been experimentally discovered that magnetic skyrmion phases can also occur in ultra-thin transition metal films at surfaces. Here we present an understanding of skyrmions in such systems based on first-principles electronic structure theory. We demonstrate that the properties of magnetic skyrmions at transition metal interfaces such as their diameter and their stability can be tuned by the structure and composition of the interface and that a description beyond a micromagnetic model is required in such systems. PMID:24893652

Dupé, Bertrand; Hoffmann, Markus; Paillard, Charles; Heinze, Stefan

2014-01-01

211

Exciton complexes in low dimensional transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter ?, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of ? is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.

Thilagam, A.

2014-08-01

212

Ductile-to-brittle transition in spallation of metallic glasses  

NASA Astrophysics Data System (ADS)

In this paper, the spallation behavior of a binary metallic glass Cu50Zr50 is investigated with molecular dynamics simulations. With increasing the impact velocity, micro-voids induced by tensile pulses become smaller and more concentrated. The phenomenon suggests a ductile-to-brittle transition during the spallation process. Further investigation indicates that the transition is controlled by the interaction between void nucleation and growth, which can be regarded as a competition between tension transformation zones (TTZs) and shear transformation zones (STZs) at atomic scale. As impact velocities become higher, the stress amplitude and temperature rise in the spall region increase and micro-structures of the material become more unstable. Therefore, TTZs are prone to activation in metallic glasses, leading to a brittle behavior during the spallation process.

Huang, X.; Ling, Z.; Dai, L. H.

2014-10-01

213

Recent Advances in Transition Metal-Catalyzed Glycosylation  

PubMed Central

Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of ?- and ?-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation. PMID:22924154

McKay, Matthew J.; Nguyen, Hien M.

2012-01-01

214

Electronic and magnetic structure of transition-metal-doped ? -hematite  

NASA Astrophysics Data System (ADS)

We investigate the electronic and magnetic structure of transition-metal-doped ? -hematite using the local density approximation with local correlations (LDA+U) . The dopants in this study are the 3d transition metals Sc-Zn and Ga and are assumed to substitute on an Fe site. The calculated net moment per substitutional impurity is found to be ?zD-zFe? , opposite to that of the replaced Fe, where zD and zFe are the numbers of valence electrons of the dopant and Fe, respectively. The dopants, D, substitute in an effective charged state D3+ except for Ti4+ and Zn2+ . In the case of Ti, the extra electron converts a neighboring Fe3+ atom to Fe2+ . In the case of Zn, the missing electron generates a relatively diffuse hole at the top of the valence band spread over neighboring O atoms.

Velev, Julian; Bandyopadhyay, A.; Butler, W. H.; Sarker, S.

2005-05-01

215

Critical Metal Phase at the Anderson Metal-Insulator Transition with Kondo Impurities  

Microsoft Academic Search

It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects.

S. Kettemann; E. R. Mucciolo; I. Varga

2009-01-01

216

Critical metal phase at the Anderson metal-insulator transition with Kondo impurities  

Microsoft Academic Search

It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects.

Eduardo Mucciolo; Stefan Kettemann; Imre Varga

2010-01-01

217

Slow dynamics due to the metal–non-metal transition in liquid mercury  

Microsoft Academic Search

The sound absorption coefficient, ?, of expanded liquid mercury has been measured at 20, 32 and 44 MHz in the temperature and pressure range up to 1600 °C and 210 MPa. Besides the critical attenuation, we have observed an anomalous increase of ? due to the relaxation process in the metal–non-metal (M–NM) transition range. Assuming a Debye-type relaxation, we show

H. Kohno; Y Kajihara; Y Hiejima; I Hirano; M Yao

2002-01-01

218

(S)TEM analysis of functional transition metal oxides  

Microsoft Academic Search

Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have

Miaofang Chi

2008-01-01

219

Scanning tunneling microscopy image of transition-metal-dichalcogenide surfaces  

Microsoft Academic Search

Scanning tunneling microscopy (STM) images of transition-metal-dichalcogenide (TMD) surfaces are studied theoretically. First, STM images of the MoS2 surface are calculated by the first-principles method with a plane-wave basis, where it has not been settled whether the first or second layer is observed in STM. It is determined theoretically that the bright spots observed in the experimental STM images correspond

Katsuyoshi Kobayashi; Jun Yamauchi

1996-01-01

220

Fermi surface nesting in several transition metal dichalcogenides  

Microsoft Academic Search

By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in

D. S. Inosov; V. B. Zabolotnyy; D. V. Evtushinsky; A. A. Kordyuk; B. Büchner; R. Follath; H. Berger; S. V. Borisenko

2008-01-01

221

Nodal liquid and s -wave superconductivity in transition metal dichalcogenides  

Microsoft Academic Search

We explore the physical properties of a unified microscopic theory for the coexistence of superconductivity and charge-density waves (CDWs) in two-dimensional transition-metal dichalcogenides. In the case of particle-hole symmetry, the elementary particles are Dirac fermions at the nodes of the charge density wave gap. When particle-hole symmetry is broken, electron (hole) pockets are formed around the Fermi surface. The superconducting

B. Uchoa; G. G. Cabrera; A. H. Castro Neto

2005-01-01

222

Charge density of group IVA transition-metal dichalcogenides  

Microsoft Academic Search

A systematic study of electronic properties of the layered group IVA transition-metal dichalcogenides TiS2, TiSe2, ZrS2 and ZrSe2 has been performed using the fully self-consistent OPW method according to the local density formalism. The X-ray form factors and the charge density are presented. The anisotropy, the bonding properties and the layer-layer interaction are analysed and discussed in detail. Also a

P. Krusius; H. Isomaki; J. von Boehm

1979-01-01

223

Band structure of group IVA transition-metal dichalcogenides  

Microsoft Academic Search

A systematic study of the electronic properties of the layered group IVA transition-metal dichalcogenides TiS2,, TiSe2, ZrS2 ZrSe2 has been performed using the ab initio fully self-consistent symmetrised OPW method according to the local density formalism (Xalpha exchange correlation). Core states, energy bands and valence densities of states are presented. The valence eigenstates are analysed in terms of Slater-Koster LCAO

H. Isomaki; J. von Boehm; P. Krusius

1979-01-01

224

Electron energy loss studies in solids; The transition metal dichalcogenides  

Microsoft Academic Search

The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed.The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX2, where M=Zr, Hf, Nb, Ta,

M. G. Bell; W. Y. Liang

1976-01-01

225

Recent developments in transition metal-catalysed spiroketalisation.  

PubMed

The spiroketal motif occurs in a wide range of biologically active natural products and represents a valuable target in medicinal chemistry and total synthesis. In recent years, innovative new synthetic methods have substantially expanded the range of potential precursors, cyclisation modes and opportunities for asymmetric catalysis and tandem processes. This Perspective aims to highlight recent rapid advances in the use of transition metal catalysis for spiroketal formation, in the context of our own investigations into gold-catalysed asymmetric spiroketalisation. PMID:25137016

Quach, Rachelle; F Chorley, Daniel; Brimble, Margaret A

2014-10-14

226

Use of Transition Metal Forms in Separation of Lipid Components  

E-print Network

charge-transfer type complexes with transition metal ions has been widely adapted in high-performance liquid chromatography (HPLC) based olefin separation(Nikolova-Damyanova, 2009). Retention of lipids in Ag-HPLC (silver ion – HPLC) depends on (i...-Damyanova, 2009). Some of the retention patterns(Nikolova-Damyanova, 2009) seen in the Ag-HPLC system are as follows: ? Retention is proportional to the number of double bonds. ? The stability is inversely proportional to the number of substituents...

Kulkarni, Sayali V

2014-08-01

227

The magnetic properties in transition metal-doped chalcopyrite semiconductors  

Microsoft Academic Search

We calculated the chemical trends of transition metal-doped chalcopyrite diluted magnetic semiconductors by use of the Korringa–Kohn–Rostoker Green's function method and the coherent potential approximation combined with the local density approximation. The ferromagnetism was stable in V- and Cr-doped chalcopyrite diluted magnetic semiconductors (DMSs). In the cases of Fe and Co doping, however, the spinglass-like state was realized. On the

T Kamatani; H Akai

2003-01-01

228

Magnetism of hexagonal 4d transition metal monolayers  

NASA Astrophysics Data System (ADS)

Using the tight-binding linear-muffin-tin-orbitals method we have calculated the spin-polarized electronic structure of 4d transition metal monolayers which have the same hexagonal structure as pseudomorphically adsorbed ones on graphite (0 0 0 1). In the 4d series, only Ru and Rh are found to be ferromagnetic with large magnetic moments of 1.9 and 1.2 ?B, respectively. Comparison with monolayers in the square structure is made.

Krüger, P.; Rakotomahevitra, A.; Moraitis, G.; Parlebas, J. C.; Demangeat, C.

1997-07-01

229

Adsorption of the formyl species on transition metal surfaces  

Microsoft Academic Search

The density functional theory (DFT) and the cluster approach were used to get information concerning the adsorption of the formyl species on transition metal surfaces. At first, the adsorption of HCO on four different sites of the Cu (111) surface was studied. An initial ?1-HCO?C conformation was considered in all cases. After optimisation, results obtained pointed towards a preferred ?1-HCO?C

J. R. B Gomes; J. A. N. F Gomes

2000-01-01

230

Structure of liquid transition and rare earth metals S. N. Khanna and F. Cyrot-Lackmann  

E-print Network

L-45 Structure of liquid transition and rare earth metals S. N. Khanna and F. Cyrot-Lackmann Groupe It is shown that the observed structure factors of transition and rare earth liquid metals can be reaso- nably. The difference is particularly large for V, Ti, and rare earth metals which are precisely the metals where

Boyer, Edmond

231

Quantum phase transition from an antiferromagnet to a spin liquid in a metal  

E-print Network

We study quantum phase transitions from easy-plane antiferromagnetic metals to paramagnetic metals in Kondo-Heisenberg lattice systems. If the paramagnetic metal is a fractionalized Fermi liquid then the universal critical ...

Grover, Tarun

232

The development of a biological interface for transition metal implants  

NASA Astrophysics Data System (ADS)

The specific goal of this research was to develop an in vitro model for a root-form endosseous dental implant that contains a periodontal ligament and that is biologically integratable into alveolar bone. This objective was based on the following two hypotheses. (1) The chemical attachment of extracellular matrix proteins to the surface of transition metals increases the number of fibroblast cells attached to the surface of the metal. (2) The chemical attachment of extracellular matrix proteins to the surface of transition metals increases the strength of the fibroblast cell attachment to the surface of the metal. The model needed to have a well-controlled surface that was reproducible. Thus, a layer of Au was deposited over a Ti base, and dithiobis(succinimidylpropionate) (DSP) a chemical containing disulfide groups was adsorbed to the Au. Next, extracellular matrix proteins which are periodontal ligament components were attached to the free end group of the chemical that was adsorbed to the Au. This surface served as an attachment substrate on which additional periodontal ligament components such as fibroblast cells could grow. From this model a new implant interface may be developed. This model was tested using the following polypeptides; collagen type I, collagen type IV, fibronectin, and poly-D-lysine. L929 cells were grown on Ti, Ti + Au, Ti + Au + polypeptide, and Ti + Au + DSP + polypeptide. After 72 hours, the live cells were stained with neutral red. The substrates were then subjected to increasing centrifugal forces. The viable stained cells were fixed onto the substrates and cells were counted. The hypotheses were proven for three polypeptides: fibronectin, collagen type I, and poly-D-lysine. The strongest attachment was found with collagen type I. Collagen type IV did not provide any advantage for attachment over uncoated transition metals.

Melton, Kim R.

233

Optical properties of transition metal oxide quantum wells  

NASA Astrophysics Data System (ADS)

Fabrication of a quantum well, a structure that confines the electron motion along one or more spatial directions, is a powerful method of controlling the electronic structure and corresponding optical response of a material. For example, semiconductor quantum wells are used to enhance optical properties of laser diodes. The ability to control the growth of transition metal oxide films to atomic precision opens an exciting opportunity of engineering quantum wells in these materials. The wide range of transition metal oxide band gaps offers unprecedented control of confinement while the strong correlation of d-electrons allows for various cooperative phenomena to come into play. Here, we combine density functional theory and tight-binding model Hamiltonian analysis to provide a simple physical picture of transition metal oxide quantum well states using a SrO/SrTiO3/SrO heterostructure as an example. The optical properties of the well are investigated by computing the frequency-dependent dielectric functions. The effect of an external electric field, which is essential for electro-optical devices, is also considered.

Lin, Chungwei; Posadas, Agham; Choi, Miri; Demkov, Alexander A.

2015-01-01

234

Luminescence in some lithiated transition metal oxide cathodes  

NASA Astrophysics Data System (ADS)

Lithiated transition metal oxides such as LiNiVO 4, Li 2MnO 3 and Li 0.4WO 3 used as cathode materials in high density lithium-ion batteries, show intense optical absorption followed by fluorescence in the region of 2-4 eV. These optical transitions can tentatively be assigned to either transition metal to oxygen charge transfer type or Jahn-Teller type local distortion. For fluorescing samples (LiNiVO 4, Li 2MnO 3 and Li 0.4WO 4), photo-induced impedance (Bode plot) data show significant enhancement in impedance under low frequency range (˜10 Hz). However, more important cathode materials such as LiCoO 2 and LiMn 2O 4 do not show either fluorescence or photo-induced impedance enhancement. The presence or absence of an optical transition in the cathode system may give some clues on the tendency for electrochemical fading.

Kalyani, P.; Jagannathan, R.; Gopukumar, S.; Lu, Chung-Hsin

235

Characterization of electrically conductive transition metal dichalcogenide lubricant films  

NASA Astrophysics Data System (ADS)

Groups VB and VIB transition metal dichalcogenides with layered structures, are intrinsic solid lubricants, and constitute a class of materials with unique and unusual properties based on their extreme anisotropy. The primary objective of this investigation was to conduct a comprehensive study on the tribological and electrical properties of burnished and sputtered transition metal dichalcogenide films, and characterize the performance under sliding electrical contact conditions. Evaluation was done using a pin-on-disc tribometer, modified to allow simultaneous monitoring of friction and contact resistance. Resistivities were measured using a static four point probe. The optimal conditions for sputter depositing transition metal dichalcogenides using the DV-602 sputtering system were determined as 10 microns of argon pressure and a rf power of 200 W. Some of the dichalcogenides did not form adherent films on stainless steel substrates under the experimental conditions. A dual source cosputtering (Cu/NbTesb2) technique is discussed, which improved the film-substrate adherence significantly, and provides a new way for the development of other thin film materials. The cosputtered Cu appears to alter the lattice parameters in the dichalcogenide crystal structure and consequently leads to improved adhesion and electrical properties. The sliding contact electrical behavior of the films was found to be related to the electronic configuration, crystal packing and structure. The group VB (Nb, Ta) metal dichalcogenides exhibited better electrical properties than the group VIB (Mo, W) metal dichalcogenide films, because of the presence of delocalized nonbonding electrons in the group VB compounds. Sputtered films with better adherence to the substrate illustrated improved friction and wear life in comparison to the burnished films. The sputtered films with greater amount of metallic character, purity, crystallinity and better adhesion reduced the contributions to the overall contact resistance. The increased metallic character of the sputtered films implies that the free electrons in the valency band are closer to the fermi level of the metal, and hence illustrate lower sliding contact resistance as compared to the burnished films. A conduction-lubrication-wear mechanism is presented, which was derived from the asperity flash temperature rise. The ohmic heating affects on the conduction-lubrication-wear mechanism of films are also discussed. The coefficient of friction, sliding contact resistance, wear rates, static four point resistivities and the asperity flash temperature estimates are presented.

Waghray, Harish C.

236

Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering  

NASA Astrophysics Data System (ADS)

Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites, which facilitate electron transfer. Here, we describe a simple and general method for covalent functionalization of two-dimensional transition metal dichalcogenide nanosheets (MoS2, WS2 and MoSe2), which does not rely on defect engineering. The functionalization reaction is instead facilitated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant, resulting in functional groups that are covalently attached to the chalcogen atoms of the transition metal dichalcogenide. The attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, and gives it strong and tunable photoluminescence and gate modulation in field-effect transistors.

Voiry, Damien; Goswami, Anandarup; Kappera, Rajesh; Silva, Cecilia De Carvalho Castro E.; Kaplan, Daniel; Fujita, Takeshi; Chen, Mingwei; Asefa, Tewodros; Chhowalla, Manish

2015-01-01

237

Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering.  

PubMed

Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites, which facilitate electron transfer. Here, we describe a simple and general method for covalent functionalization of two-dimensional transition metal dichalcogenide nanosheets (MoS2, WS2 and MoSe2), which does not rely on defect engineering. The functionalization reaction is instead facilitated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant, resulting in functional groups that are covalently attached to the chalcogen atoms of the transition metal dichalcogenide. The attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, and gives it strong and tunable photoluminescence and gate modulation in field-effect transistors. PMID:25515889

Voiry, Damien; Goswami, Anandarup; Kappera, Rajesh; Silva, Cecilia de Carvalho Castro E; Kaplan, Daniel; Fujita, Takeshi; Chen, Mingwei; Asefa, Tewodros; Chhowalla, Manish

2015-01-01

238

Lattice dynamics of transition metals in real space  

NASA Astrophysics Data System (ADS)

The effective ion-ion interaction is written as the sum of s-s, d-d attractive and d-d overlap interactions. The free electron part of the pair potential is obtained in second-order perturbation theory using a two-parameter Heine-Abarenkov model potential and a modified Lindhard dielectric function as has been suggested by Hafner and Heine. The overlap d states of different ions and the effect of s-d hybridization obtained by Wills and Harrison are considered. The temperature dependence has been included in an approxmate manner. It is then used to calculate the phonon spectra of a number of transition metals in the FCC phase. The agreement between predicted and observed values is found to be reasonably good for all the metals. The binding energy and elastic constants of these metals are also calculated.

Singh, N.; Yadav, B. S.

1994-09-01

239

N-heterocyclic carbene-phosphinidyne transition metal complexes.  

PubMed

The N-heterocyclic carbene-phosphinidene adduct IPr?PSiMe3 is introduced as a synthon for the preparation of terminal carbene-phosphinidyne transition metal complexes of the type [(IPr?P)MLn ] (MLn =(?(6) -p-cymene)RuCl) and (?(5) -C5 Me5 )RhCl). Their spectroscopic and structural characteristics, namely low-field (31) P?NMR chemical shifts and short metal-phosphorus bonds, show their similarity with arylphosphinidene complexes. The formally mononegative IPr?P ligand is also capable of bridging two or three metal atoms as demonstrated by the preparation of bi- and trimetallic RuAu, RhAu, Rh2 , and Rh2 Au complexes. PMID:25287885

Doddi, Adinarayana; Bockfeld, Dirk; Bannenberg, Thomas; Jones, Peter G; Tamm, Matthias

2014-12-01

240

Method for treating rare earth-transition metal scrap  

DOEpatents

Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a prefused, rare earth fluoride-bearing flux of CaF.sub.2, CaCl.sub.2 or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy.

Schmidt, Frederick A. (Ames, IA); Peterson, David T. (Ames, IA); Wheelock, John T. (Nevada, IA); Jones, Lawrence L. (Des Moines, IA)

1992-12-29

241

Transition-metal prion protein attachment: Competition with copper  

NASA Astrophysics Data System (ADS)

Prion protein, PrP, is a protein capable of binding copper ions in multiple modes depending on their concentration. Misfolded PrP is implicated in a group of neurodegenerative diseases, which include ``mad cow disease'' and its human form, variant Creutzfeld-Jacob disease. An increasing amount of evidence suggests that attachment of non-copper metal ions to PrP triggers transformations to abnormal forms similar to those observed in prion diseases. In this work, we use hybrid Kohn-Sham/orbital-free density functional theory simulations to investigate copper replacement by other transition metals that bind to PrP, including zinc, iron and manganese. We consider all known copper binding modes in the N-terminal domain of PrP. Our calculations identify modes most susceptible to copper replacement and reveal metals that can successfully compete with copper for attachment to PrP.

Hodak, Miroslav; Bernholc, Jerry

2012-02-01

242

Method for treating rare earth-transition metal scrap  

DOEpatents

Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a rare earth fluoride-bearing flux of CaF[sub 2], CaCl[sub 2] or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy. 3 figs.

Schmidt, F.A.; Peterson, D.T.; Wheelock, J.T.; Jones, L.L.

1992-12-29

243

Thermophysical Property Measurements of Silicon-Transition Metal Alloys  

NASA Technical Reports Server (NTRS)

Metals and metallic alloys often have high melting temperatures and highly reactive liquids. Processing reactive liquids in containers can result in significant contamination and limited undercooling. This is particularly true for molten silicon and it alloys. Silicon is commonly termed "the universal solvent". The viscosity, surface tension, and density of several silicon-transition metal alloys were determined using the Electrostatic Levitator system at the Marshall Space Flight Center. The temperature dependence of the viscosity followed an Arrhenius dependence, and the surface tension followed a linear temperature dependence. The density of the melts, including the undercooled region, showed a linear behavior as well. Viscosity and surface tension values were obtain for several of the alloys in the undercooled region.

Banish, R. Michael; Erwin, William R.; Sansoucie, Michael P.; Lee, Jonghyun; Gave, Matthew A.

2014-01-01

244

Critical Transitions in Early Embryonic Aortic Arch Patterning and Hemodynamics  

PubMed Central

Transformation from the bilaterally symmetric embryonic aortic arches to the mature great vessels is a complex morphogenetic process, requiring both vasculogenic and angiogenic mechanisms. Early aortic arch development occurs simultaneously with rapid changes in pulsatile blood flow, ventricular function, and downstream impedance in both invertebrate and vertebrate species. These dynamic biomechanical environmental landscapes provide critical epigenetic cues for vascular growth and remodeling. In our previous work, we examined hemodynamic loading and aortic arch growth in the chick embryo at Hamburger-Hamilton stages 18 and 24. We provided the first quantitative correlation between wall shear stress (WSS) and aortic arch diameter in the developing embryo, and observed that these two stages contained different aortic arch patterns with no inter-embryo variation. In the present study, we investigate these biomechanical events in the intermediate stage 21 to determine insights into this critical transition. We performed fluorescent dye microinjections to identify aortic arch patterns and measured diameters using both injection recordings and high-resolution optical coherence tomography. Flow and WSS were quantified with 3D computational fluid dynamics (CFD). Dye injections revealed that the transition in aortic arch pattern is not a uniform process and multiple configurations were documented at stage 21. CFD analysis showed that WSS is substantially elevated compared to both the previous (stage 18) and subsequent (stage 24) developmental time-points. These results demonstrate that acute increases in WSS are followed by a period of vascular remodeling to restore normative hemodynamic loading. Fluctuations in blood flow are one possible mechanism that impacts the timing of events such as aortic arch regression and generation, leading to the variable configurations at stage 21. Aortic arch variations noted during normal rapid vascular remodeling at stage 21 identify a temporal window of increased vulnerability to aberrant aortic arch morphogenesis with the potential for profound effects on subsequent cardiovascular morphogenesis. PMID:23555940

Kowalski, William J.; Dur, Onur; Wang, Yajuan; Patrick, Michael J.; Tinney, Joseph P.; Keller, Bradley B.; Pekkan, Kerem

2013-01-01

245

Synthesis and X-ray characterization of 3D transition metal intercalates of transition metal dichalcogenides and their superlattices  

NASA Astrophysics Data System (ADS)

New superlattices of 3d transition metal intercalates of transition metal dichalcogenides have been synthesized by deposition and annealing of layered reactants. Sheets of atoms are the building blocks of transition metal dichalcogenides. Since the inter-sheet bonding is weak, guest atoms may be introduced between the sheets (intercalation) and sheets of differing atomic species may be stacked one atop the other (forming a superlattice), and, by intermingling such modifications, many different bulk materials may potentially be synthesized. FexNbSe 2/FexTiSe2 and CrxNbSe2/Cr xTiSe2 superlattices and NixMoSe2 intercalates have been grown and characterized with X-ray diffraction. These materials display anticipated atomic structures and are stable in a Nitrogen atmosphere at temperatures up to 500°C, testifying to the synthetic feasibility of this novel family of superlattices. The 3d transition metal intercalates order magnetically and exhibit other electron correlation effects which could potentially be tailored by rational superlattice design. However, preliminary investigations have not found evidence of such effects in the new superlattices; presumably, structural defects in the samples are inhibiting these phenomena. We demonstrate that standard X-ray diffraction structure analysis techniques are generally inapplicable to superlattices; these new materials are no exception. An original procedure for structural refinement, utilizing both conjugate gradient and simulated annealing optimization methods, is used to generate a model superlattice having a calculated X-ray diffraction profile which matches experimental measurements. This procedure satisfactorily fits X-ray diffraction peak shapes over a large range of scattering angles, and is demonstrated in determining the distribution of crystal sizes, the distribution of defects, and the average superlattice unit cell parameters for an (Fe0.3NbSe2)8/(Fe0.3TiSe 2)8 sample.

Hughes, Thomas Abbott

1998-12-01

246

Transitions in critical size in metal (100) and metal (111) homoepitaxy  

Microsoft Academic Search

Transitions in the critical island size i in submonolayer growth from i = 1 to i = 2 and from i = 1 to i = 3, corresponding to homoepitaxial growth on metal (111) and (100) surfaces, are studied using kinetic Monte Carlo simulations of a restricted pairbond model, both with and without island relaxation, and are compared with rate-equation

Jacques G. Amar; Fereydoon Family

1997-01-01

247

Transition radiation in metal-metal multilayer nanostructures as a medical source of hard x-ray radiation  

E-print Network

Transition radiation in metal-metal multilayer nanostructures as a medical source of hard x; accepted 20 July 2006; published online 31 August 2006 We show that a periodic metal-metal multilayer to medicine 30­50 keV changes dramatically compared with that for soft x-ray radiation. We show that one

Kaplan, Alexander

248

State Efforts to Meet the Early Childhood Transition Requirements of IDEA. inForum  

ERIC Educational Resources Information Center

In March 2008, the National Early Childhood Transition Initiative released a document developed collaboratively over several months titled "Designing and Implementing Effective Early Childhood Transition Processes". The document was created as a resource for improving state and local performance on the State Performance Plans (SPP) and Annual…

Muller, Eve; Whaley, Kathy; Rous, Beth

2009-01-01

249

Metal-Insulator Transition of the Hubbard Chain with Next-Nearest-Neighbor Hopping Partially Ferromagnetic Metal toward Mott Insulator  

Microsoft Academic Search

Metal-insulator transition in the ground state of the Hubbard chain with the next-nearest-neighbor hopping is studied by exact diagonalization method. This model shows the partially ferromagnetic metal near the Mott insulator for large Coulomb repulsion while the model reveals the non-magnetic metal for small but finite Coulomb repulsion. We find the transition between the partially ferromagnetic metal and the Mott

Hiroki Nakano; Yoshinori Takahashi

2004-01-01

250

New transition-metal-dependent DNAzymes as efficient endonucleases and as selective metal biosensors.  

PubMed

Like proteins and RNA molecules, many DNA molecules have now been shown to catalyze a variety of reactions and are thus called DNAzymes. With limited building blocks, DNAzymes need to recruit other cofactors in order to match other enzymes in terms of reaction diversity and catalytic efficiency. Several unique properties make transition-metal ions an ideal choice of cofactor for DNAzymes. Indeed, new DNAzymes that bind transition-metal ions with high affinity and selectivity have been obtained through the use of a powerful combinatorial biology tool called in vitro selection. This accomplishment now makes it possible to obtain different classes of metallo-DNAzymes in the laboratory within a short period of time. It also offers a rare opportunity to compare and contrast structural and functional properties of metal-binding sites in proteins and in DNAzymes. The resulting transition-metal-dependent DNAzymes have displayed high activity toward cleavage of DNA and RNA and thus hold promise for their biochemical and pharmaceutical applications. Finally, the use of DNAzymes as a new class of highly sensitive and selective biosensors for metal ions has been demonstrated recently. PMID:12561102

Lu, Yi

2002-10-18

251

New transition-metal-dependent DNAzymes as efficient endonucleases and as selective metal biosensors.  

PubMed

Like proteins and RNA molecules, many DNA molecules have now been shown to catalyze a variety of reactions and are thus called DNAzymes. With limited building blocks, DNAzymes need to recruit other cofactors in order to match other enzymes in terms of reaction diversity and catalytic efficiency. Several unique properties make transition-metal ions an ideal choice of cofactor for DNAzymes. Indeed, new DNAzymes that bind transition-metal ions with high affinity and selectivity have been obtained through the use of a powerful combinatorial biology tool called in vitro selection. This accomplishment now makes it possible to obtain different classes of metallo-DNAzymes in the laboratory within a short period of time. It also offers a rare opportunity to compare and contrast structural and functional properties of metal-binding sites in proteins and in DNAzymes. The resulting transition-metal-dependent DNAzymes have displayed high activity toward cleavage of DNA and RNA and thus hold promise for their biochemical and pharmaceutical applications. Finally, the use of DNAzymes as a new class of highly sensitive and selective biosensors for metal ions has been demonstrated recently. PMID:12362396

Lu, Yi

2002-10-18

252

Catalytic Applications of Early\\/Late Heterobimetallic Complexes  

Microsoft Academic Search

Transition metal complexes that combine electronically different metals are most readily designed by combining an early transition metal and a late transition metal, typically via a bridging ligand framework. These so-called “early\\/late” heterobimetallic complexes have recently proven active in a number of catalytic transformations that are not accessible with analogous monometallic complexes. This review discusses recent contributions to catalytic and

Benjamin G. Cooper; J. Wesley Napoline; Christine M. Thomas

2012-01-01

253

Transition metal complexes of isonicotinic acid (2-hydroxybenzylidene)hydrazide.  

PubMed

A new series of transition metal complexes of Schiff base isonicotinic acid (2-hydroxybenzylidene)hydrazide, HL, have been synthesized. The Schiff base reacted with Cu(II), Ni(II), Co(II), Mn(II), Fe(III) and UO2(II) ions as monobasic tridentate ligand to yield mononuclear complexes of 1:2 (metal:ligand) except that of Cu(II) which form complex of 1:1 (metal:ligand). The ligand and its metal complexes were characterized by elemental analyses, IR, UV-vis, mass and 1H NMR spectra, as well as magnetic moment, conductance measurements, and thermal analyses. All complexes have octahedral configurations except Cu(II) complex which has an extra square planar geometry distorted towards tetrahedral. While, the UO2(II) complex has its favour hepta-coordination. The ligand and its metal complexes were tested against one strain Gram +ve bacteria (Staphylococcus aureus), Gram -ve bacteria (Escherichia coli), and Fungi (Candida albicans). The tested compounds exhibited higher antibacterial activities. PMID:17728178

Abou-Melha, Khlood S

2008-06-01

254

Transition metal complexes of isonicotinic acid (2-hydroxybenzylidene)hydrazide  

NASA Astrophysics Data System (ADS)

A new series of transition metal complexes of Schiff base isonicotinic acid (2-hydroxybenzylidene)hydrazide, HL, have been synthesized. The Schiff base reacted with Cu(II), Ni(II), Co(II), Mn(II), Fe(III) and UO 2(II) ions as monobasic tridentate ligand to yield mononuclear complexes of 1:2 (metal:ligand) except that of Cu(II) which form complex of 1:1 (metal:ligand). The ligand and its metal complexes were characterized by elemental analyses, IR, UV-vis, mass and 1H NMR spectra, as well as magnetic moment, conductance measurements, and thermal analyses. All complexes have octahedral configurations except Cu(II) complex which has an extra square planar geometry distorted towards tetrahedral. While, the UO 2(II) complex has its favour hepta-coordination. The ligand and its metal complexes were tested against one strain Gram +ve bacteria ( Staphylococcus aureus), Gram -ve bacteria (Escherichia coli) , and Fungi ( Candida albicans). The tested compounds exhibited higher antibacterial activities.

Abou-Melha, Khlood S.

2008-06-01

255

Various methods for determining the critical metallic volume fraction ? c at the metal-insulator transition  

NASA Astrophysics Data System (ADS)

The metal-insulator transition in composite Al-Ge films has been extensively studied experimentally. Using the superconducting properties of percolating clusters composed of small aluminum grains in the presence of a magnetic field, the critical metallic volume fraction ? c(sc) for the metal-insulator transition has been uniquely determined. The low temperature electrical conductivity data are examined using a number of criteria for ? c. Some of these criteria are quite successful, yielding ? c's to within one percent of ? c(sc), the definitive value. However, one widely used criterion involving the fitting of the low temperature conductivity data to a T{1}/{2} dependence, which results from the electron-electron interaction theory, was found to yield a very different value of ? c. Other methods and/or criteria, using room temperature conductivity data to determine ? c, are evaluated. These also yield ? c values that are within 1% of ? c(sc).

Rosenbaum, Ralph L.; Slutzky, Michael; McLachlan, D. S.; Möbius, Arnulf

1994-06-01

256

Critical Metal Phase at the Anderson Metal-Insulator Transition with Kondo Impurities  

NASA Astrophysics Data System (ADS)

It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects. We show that multifractal correlations open local pseudogaps at the Fermi energy at some random positions in space. When dilute magnetic impurities are at these locations, Kondo screening is strongly suppressed. When the exchange coupling J is smaller than a certain value J*, the metal-insulator transition point extends to a critical region in the disorder strength parameter and to a band of critical states.

Kettemann, S.; Mucciolo, E. R.; Varga, I.

2009-09-01

257

Critical metal phase at the Anderson metal-insulator transition with Kondo impurities  

NASA Astrophysics Data System (ADS)

It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects. We show that multifractal correlations open local pseudogaps at the Fermi energy at some random positions in space. When dilute magnetic impurities are at these locations, Kondo screening is strongly suppressed. When the exchange coupling J is smaller than a certain value J^*, the metal-insulator transition point extends to a critical region in the disorder strength parameter and to a band of critical states. The width of this critical region increases with a power of the concentration of magnetic impurities. [S. Kettemann, E. R. Mucciolo, and I. Varga, Phys. Rev. Lett. 103, 126401 (2009).

Mucciolo, Eduardo; Kettemann, Stefan; Varga, Imre

2010-03-01

258

Mechanically induced metal-insulator transition in carbyne.  

PubMed

First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations eliminate the Peierls distortion in the mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, the competition between the zero-point vibrations and mechanical strain determines a switch in symmetry resulting in the transition from metallic state to a dielectric, with a small effective mass and a high carrier mobility. In any practical realization, it is important that the effect is also chemically modulated by the choice of terminating groups. These findings are promising for applications such as electromechanical switching and band gap tuning via strain, and besides carbyne itself, they directly extend to numerous other systems that show Peierls distortion. PMID:24991984

Artyukhov, Vasilii I; Liu, Mingjie; Yakobson, Boris I

2014-08-13

259

Semiconductor-metal transition induced by nanoscale stabilization.  

PubMed

The structure of tin (Sn) nanoparticles as function of size and temperature has been studied using density functional theory and thermodynamic considerations. It is known that bulk Sn undergoes a transition from the semiconducting ?-phase to the metallic ?-phase at temperatures higher than 13.2 °C under atmospheric pressure. Here we show that, independent of temperature, Sn nanoparticles smaller than 8 nm diameter always crystallize in the ?-phase structure in thermodynamic equilibrium, and up to a size of 40 nm of the Sn nanoparticles this metallic phase is stable at all reasonable ambient temperatures (?-40 °C). The transition to the metallic phase is caused by nanoscale stabilization due to the lower surface energies of the ? phase. This study suggests that the atomic structure and conductivity of nanostructured Sn anodes can change dramatically with size and temperature. This finding has implication for understanding the performance of future Li-based batteries since Sn nanostructures are considered as one of the most promising anode materials, but the mechanism of nanoscale stabilization might be used as a design strategy for other materials. PMID:25626452

Hörmann, Nicolas G; Gross, Axel; Kaghazchi, Payam

2015-02-10

260

Nature of metal-nonmetal transition in metal-ammonia solutions. II. From uniform metallic state to inhomogeneous electronic microstructure.  

PubMed

Applying semianalytical models of nonideal plasma, we evaluate the behavior of the metallic phase in metal-ammonia solutions (MAS). This behavior is mainly controlled by the degenerate electron gas, which remains stable down to 5 MPM due to high solvent polarizability and strong dielectric screening of solvated ions. Comparing the behavior of the metallic state with those of localized solvated electrons, we have estimated the miscibility gap Delta n for various alkali metals and found Delta n(Na)>Delta n(K). It is rather narrow in Rb-NH3 and does not occur in Cs-NH3 solutions, which is in full agreement with the experiments. The case of Li is discussed separately. The difference calculated in the excess free energies of the metallic and nonmetallic phases is in the order of kBT, yielding a thermally fluctuating mixed state at intermediate metal concentrations. It results in a continuous metal-nonmetal (MNM) transition above the consolute point Tc and a phase separation below Tc. We propose a criterion for the MNM transition which may be attributed to the line of the maximum of compressibility above Tc. This line crosses the spinodal one at the critical temperature. Finally, we assert that a new electronic phase similar to microemulsion should also arise between the spinodal and the binodal lines. PMID:18412455

Chuev, Gennady N; Quémerais, Pascal

2008-04-14

261

Parents' Marital Distress, Divorce, and Remarriage: Links with Daughters' Early Family Formation Transitions  

ERIC Educational Resources Information Center

The authors used data from the Add Health study to estimate the effects of parents' marital status and relationship distress on daughters' early family formation transitions. Outcomes included traditional transitions (marriage and marital births) and nontraditional transitions (cohabitation and nonmarital births). Relationship distress among…

Amato, Paul R.; Kane, Jennifer B.

2011-01-01

262

Semiconductor-metal and metal-semiconductor transitions in twisting graphene nanoribbons  

NASA Astrophysics Data System (ADS)

The electronic structure and transport properties of twisting graphene nanoribbons (TGNRs) are systematically investigated using the tight-binding model and the non-equilibrium Green's function method. We show that the energy gap and conductance around the Fermi energy can be reversibly modulated. Armchair TGNRs (ATGNRs) can be either metallic or semiconducting depending on the widths and the twist angles of the GNRs. Semiconductor-metal and metal-semiconductor transitions are observed in ATGNRs for N=3i+1 (where i is an integer and N is the number of atoms along the width of the nanoribbon) and N=3i+2, respectively. Narrow ATGNRs are semiconductors for N=3i, whereas zigzag TGNRs (ZTGNRs) are metallic regardless of the width and distortion of the GNRs.

Xu, Ning; Huang, Bolong; Li, Jianfu; Wang, Baolin

2015-01-01

263

Metal-insulator transition in pyrochlore Eu2Ir2O7 studied by infrared spectroscopy  

NASA Astrophysics Data System (ADS)

The large family of pyrochlores with formula A2B2C7 attracted a lot of early attention due to strong geometric magnetic frustration. Recent band structure calculations predict that the iridate pyrochlores A2Ir2O7 may have nontrivial topological states. We will report the results of an infrared spectroscopic study of the metal-insulator transition in Eu2Ir2O7 single crystal and Y2Ir2O7 polycrystal. We will report the broad band IR reflection as a function of temperature for an overview of the M-I transition and the low frequency transmission which is more sensitive for detection of a 10 meV gap[1] and other possible excitations. We will discuss possible implications of the semimetal Weyl states. [1] J.J. Ishikawa et al., Phys. Rev. B 85, 245109 (2012).

Sushkov, Andrei; Drew, Dennis; Ishikawa, Jun; Nakatsuji, Satoru; Luo, Xuan; Cheong, Sang-Wook

2013-03-01

264

Computer Simulations of Wetting Transitions of Ne on Alkali Metals  

NASA Astrophysics Data System (ADS)

The wetting properties of Ne on alkali metal surfaces are investigated using grand canonical Monte Carlo simulations. The values of the well depth of the gas-surface interaction potential considered here correspond roughly to those of Ne on alkali metals(A. Chizmeshya, M. W. Cole, and E. Zaremba, J. Low Temp. Phys., in press.). For a range of temperatures extending from slightly above the bulk triple point of the gas to 0.95 of the critical temperature (depending on the interaction strength), we find (apparently first order) prewetting transitions. The simulation results are consistent (as far as they go) with the behavior studied experimentally(G. B. Hess, M. J. Sabatini and M. H. W. Chan, Phys. Rev. Lett. 78, 1739 (1997).).

Stan, G.; Bojan, M. J.; Cole, M. W.; Steele, W. A.

1998-03-01

265

Role of transition metals in sclerotization of biological tissue.  

PubMed

Biomimetics seeks to elucidate the biochemical and structural principles governing functionality in biological systems and exploit them in the design of synthetic materials. Although the lure has been great, breakthroughs have been rare. The overwhelming complexity of chemical compositions and structural motifs in biology renders the task of "reverse engineering" a formidable challenge. The challenge is amplified by the inherent limitations in probing the effects of a single test variable, keeping others fixed. Here a novel methodology is presented for probing the effects of various transition metals on the hardness and stiffness of a single biological system, notably the jaws of the marine polychaete annelid Nereis virens. Not only is the exceptionally high efficacy of Zn, Cu, and Mn in hardening demonstrated, but also that metal selection is not restricted to that which occurs naturally in the targeted system. PMID:18653388

Broomell, C C; Zok, F W; Waite, J H

2008-11-01

266

Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice  

NASA Astrophysics Data System (ADS)

The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems.

Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

2014-04-01

267

Development of microstrain in aged lithium transition metal oxides.  

PubMed

Cathode materials with high energy density for lithium-ion batteries are highly desired in emerging applications in automobiles and stationary energy storage for the grid. Lithium transition metal oxide with concentration gradient of metal elements inside single particles was investigated as a promising high-energy-density cathode material. Electrochemical characterization demonstrated that a full cell with this cathode can be continuously operated for 2500 cycles with a capacity retention of 83.3%. Electron microscopy and high-resolution X-ray diffraction were employed to investigate the structural change of the cathode material after this extensive electrochemical testing. It was found that microstrain developed during the continuous charge/discharge cycling, resulting in cracking of nanoplates. This finding suggests that the performance of the cathode material can be further improved by optimizing the concentration gradient to minimize the microstrain and to reduce the lattice mismatch during cycling. PMID:24960550

Lee, Eung-Ju; Chen, Zonghai; Noh, Hyung-Ju; Nam, Sang Cheol; Kang, Sung; Kim, Do Hyeong; Amine, Khalil; Sun, Yang-Kook

2014-08-13

268

Metal-insulator Transition by Holographic Charge Density Waves  

E-print Network

We construct a gravity dual for charge density waves (CDW) in which the translational symmetry along one spatial direction is spontaneously broken. Our linear perturbation calculation on the gravity side produces the frequency dependence of the optical conductivity, which exhibits the two familiar features of charge density waves, namely the pinned collective mode and gapped single-particle excitation. These two features indicate that our gravity dual also provides a new mechanism to implement the metal to insulator phase transition by CDW, which is further supported by the fact that d.c. conductivity decreases with the decreased temperature below the critical temperature.

Yi Ling; Chao Niu; Jianpin Wu; Zhuoyu Xian; Hongbao Zhang

2014-08-06

269

Valley photothermoelectric effects in transition-metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We theoretically investigate photothermoelectric effects, i.e., the photoenabled versions of the Seebeck and Nernst effects in monolayer transition metal dichalcogenides (TMDCs) using semiclassical transport theory. We find that monolayer TMDCs subjected to circular polarized light show anomalous thermoelectric properties as a result of strong spin-orbit interaction and broken inversion symmetry. The Seebeck coefficient for photogenerated carriers is relatively large and changes its sign at a critical carrier density. In addition, a nontrivial photo-Nernst effect emerges in the absence of an external magnetic field or magnetic moments.

Konabe, Satoru; Yamamoto, Takahiro

2014-08-01

270

Band Structures of Transition-Metal-Dichalcogenide Layer Compounds  

Microsoft Academic Search

The nonrelativistic augmented-plane-wave (APW) method is applied to calculate the electronic band structures of several transition-metal-dichalcogenide (TX2) layer compounds, including materials with the C 6 (1T-HfS2,1T-TaS2), C 27 (2H-TaS2,2H-NbSe2), and C 7 (2H-MoS2) structure types. These calculations involve crystal potentials that are derived from neutral-atom charge densities. The results of these calculations confirm that the group-IVB (1T-HfS2) and group-VIB (2H-MoS2)

L. F. Mattheiss

1973-01-01

271

Theoretical study of electron correlation effects in transition metal dimers  

NASA Technical Reports Server (NTRS)

Introduction of partially localized orbitals is shown to reduce the number of terms needed to describe the bonding in transition metal clusters. Using this formalism, it is possible to compute the various intra- and inter-atomic electron correlation contributions to the bond energy. Calculations demonstrate the relative importance of several kinds of electron correlation terms involving the 3p, 3d, and 4s electrons. Improved interaction potentials are obtained for the dimers V(2) and Cr(2) when additional correlation is added to the CAS SCF results of Walch, Bauschlicher, Roos, and Nelin (1983).

Das, G. P.; Jaffe, R. L.

1984-01-01

272

Metal-insulator transition by holographic charge density waves.  

PubMed

We construct a gravity dual for charge density waves (CDWs) in which the translational symmetry along one spatial direction is spontaneously broken. Our linear perturbation calculation on the gravity side produces the frequency dependence of the optical conductivity, which exhibits the two familiar features of CDWs, namely, the pinned collective mode and gapped single-particle excitation. These two features indicate that our gravity dual also provides a new mechanism to implement the metal to insulator phase transition by CDWs, which is further confirmed by the fact that dc conductivity decreases with the decreased temperature below the critical temperature. PMID:25215974

Ling, Yi; Niu, Chao; Wu, Jian-Pin; Xian, Zhuo-Yu; Zhang, Hongbao

2014-08-29

273

Tetrathiafulvalene-annulated phenanthroline and its complexes with transition metals  

NASA Astrophysics Data System (ADS)

Recent interest in designing multifunctional transition metal complexes that incorporate the redox-active tetrathiafulvalene (TTF) molecule stems from the perspective of embedding specific localized properties of transition metal ions into TTF-containing substructures characterized by extended delocalized conduction pathways. This dissertation reports on the synthesis of tetrathiafulvalene, a ubiquitous component of organic conductors, fused directly to 1,10-phenanthroline, one of the most popular ligands in transition metal chemistry. We have devised and implemented a synthetic pathway to this unique ligand, which then was used to obtain complexes with Ru(II) and Fe(II) ions. Chapter 1 of this dissertation provides an introduction to the chemistry of TTF and its metal complexes. Chapter 2 describes the most important methods of characterization employed in this research project. Chapter 3 is devoted to the preparation and properties of the TTF-annulated phenanthroline ligands. Chapter 4 describes a detailed study of a Ru(II) complex of TTF phenanthroline, including its structural, photophysical, and electrochemical properties. [Ru(bpy) 2(edt-TTF-phen)](PF6)2 exhibits a unique helical packing of molecules in the crystal structure. In contrast to its TTF-free analogues, this complex contains a TTF-centered highest occupied molecular orbital, and the electrochemical data show that the first two oxidations of the complex are TTF-based. Examination of the photophysical properties revealed that this Ru(II) complex experiences excited-state luminescence quenching via reductive electron transfer from the TTF to the Ru3+ center, which opens up a path for non radiative relaxation to the ground state through the low-lying intra-ligand charge transfer state. We also describe an unusual photoreactivity of this complex that leads to the cleavage of the central C=C bond of the TTF unit in solution. Such reactivity was monitored by the evolution of NMR, UV-visible, emission, and transient absorption spectra. Chapter 5 describes spin-crossover behavior of [Fe(NCS)2(L) 2] (L = edt-TTF-phen or hexS2-TTF-phen) and parent Fe(II) complexes. The complex with hexyl substituents is highly soluble and exhibits reversible oxidations centered on the TTF fragments. The observed spin transition was elucidated with variable-temperature Mossbauer and infrared spectroscopies.

Keniley, Lawrence Keith, Jr.

274

Rare earth-transition metal scrap treatment method  

DOEpatents

Rare earth-transition metal (e.g. iron) scrap (e.g. Nd-Fe-B scrap) is melted to reduce the levels of tramp oxygen and nitrogen impurities therein. The tramp impurities are reduced in the melt by virtue of the reaction of the tramp impurities and the rare earth to form dross on the melt. The purified melt is separated from the dross for reuse. The oxygen and nitrogen of the melt are reduced to levels acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets.

Schmidt, Frederick A. (Ames, IA); Peterson, David T. (Ames, IA); Wheelock, John T. (Nevada, IA); Jones, Lawrence L. (Des Moines, IA); Lincoln, Lanny P. (Woodward, IA)

1992-02-11

275

TOPICAL REVIEW: Structural changes and the metal-non-metal transition in supercritical fluids  

NASA Astrophysics Data System (ADS)

Energy-dispersive x-ray diffraction (XD), small-angle x-ray scattering (SAXS) and x-ray absorption fine-structure (XAFS) measurements for metallic fluids up to the supercritical region were carried out using synchrotron radiation. We obtained the structure factor S(k) and the pair distribution functions g(r) for expanded fluid Hg from the liquid to the dense vapour region including the metal-non-metal (M-NM) transition region in the density range from 13.6 to 1.9 g cm-3 by means of XD measurements at SPring-8 in Japan. The density variations of the interatomic distance (r1) and coordination number (N1) obtained are discussed in relation to the M-NM transition in fluid Hg. To investigate structural change in the semiconductor-metal (SC-M) transition in expanded fluid Se, XD measurements, at SPring-8, and XAFS measurements, at the European Synchrotron Radiation Facility (ESRF) in France, were carried out at high temperatures and high pressures. It was found that the twofold-coordinated chain structure is preserved and contraction of the covalent bond occurs on the SC-M transition. XAFS measurements for dense Se vapour near the critical point were also carried out to study how dimers in the rarefied vapour condense to the metallic fluid. SAXS measurements were carried out to obtain information on the density fluctuation of fluid Se near the critical point. On the basis of the structural data obtained by XD, XAFS and SAXS measurements for fluid Se, we discuss how the density fluctuation affects the local structure and electronic properties of fluid Se near the critical point.

Tamura, Kozaburo; Inui, Masanori

2001-05-01

276

The Timing of School Transitions and Early Adolescent Problem Behavior  

ERIC Educational Resources Information Center

This longitudinal study investigates whether rural adolescents who transition to a new school in sixth grade have higher levels of risky behavior than adolescents who transition in seventh grade. Our findings indicate that later school transitions had little effect on problem behavior between sixth and ninth grades. Cross-sectional analyses found…

Lippold, Melissa A.; Powers, Christopher J.; Syvertsen, Amy K.; Feinberg, Mark E.; Greenberg, Mark T.

2013-01-01

277

Glutathione and Transition-Metal Homeostasis in Escherichia coli?  

PubMed Central

Glutathione (GSH) and its derivative phytochelatin are important binding factors in transition-metal homeostasis in many eukaryotes. Here, we demonstrate that GSH is also involved in chromate, Zn(II), Cd(II), and Cu(II) homeostasis and resistance in Escherichia coli. While the loss of the ability to synthesize GSH influenced metal tolerance in wild-type cells only slightly, GSH was important for residual metal resistance in cells without metal efflux systems. In mutant cells without the P-type ATPase ZntA, the additional deletion of the GSH biosynthesis system led to a strong decrease in resistance to Cd(II) and Zn(II). Likewise, in mutant cells without the P-type ATPase CopA, the removal of GSH led to a strong decrease of Cu(II) resistance. The precursor of GSH, ?-glutamylcysteine (?EC), was not able to compensate for a lack of GSH. On the contrary, ?EC-containing cells were less copper and cadmium tolerant than cells that contained neither ?EC nor GSH. Thus, GSH may play an important role in trace-element metabolism not only in higher organisms but also in bacteria. PMID:18539744

Helbig, Kerstin; Bleuel, Corinna; Krauss, Gerd J.; Nies, Dietrich H.

2008-01-01

278

Effect of transition metal dopants on mechanical properties and biocompatibility of zirconia ceramics.  

PubMed

In this study, the effect of transition metal dopants, originally added as colouring agents, on the mechanical properties and biocompatibility of sintered zirconia was investigated. This study confirmed that transition metal dopants could have a slight detrimental effect on the mechanical properties of zirconia. The addition of metal dopants did not affect the adhesion and proliferation of gingival fibroblasts. PMID:23862482

Oh, Gye-Jeong; Park, Sang-Won; Yun, Kwi-Dug; Lim, Hyun-Pil; Son, Hye-Ju; Koh, Jeong-Tae; Lee, Kyung-Ku; Lee, Doh-Jae; Lee, Kwang-Min; Fisher, John G

2013-06-01

279

Transition-metal-catalyzed oxidation of metallic Sn in NiO/SnO2 nanocomposite.  

PubMed

It is well accepted that metallic tin as a discharge (reduction) product of SnO(x) cannot be electrochemically oxidized below 3.00?V versus Li(+)/Li(0) due to the high stability of Li2O, though a similar oxidation can usually occur for a transition metal formed from the corresponding oxide. In this work, nanosized Ni2 SnO4 and NiO/SnO2 nanocomposite were synthesized by coprecipitation reactions and subsequent heat treatment. Owing to the catalytic effect of nanosized metallic nickel, metallic tin can be electrochemically oxidized to SnO2 below 3.00?V. As a result, the reversible lithium-storage capacities of the nanocomposite reach 970?mAh?g(-1) or above, much higher than the theoretical capacity (ca. 750?mAh?g(-1)) of SnO2, NiO, or their composites. These findings extend the well-known electrochemical conversion reaction to non-transition-metal compounds and may have important applications, for example, in constructing high-capacity electrode materials and efficient catalysts. PMID:24648283

Hua, Chunxiu; Fang, Xiangpeng; Wang, Zhaoxiang; Chen, Liquan

2014-04-25

280

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.  

PubMed

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques. PMID:24776648

Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I; Cuong, Nguyen Thanh; Otani, Minoru; Okada, Susumu; Lupini, Andrew R; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J; Yan, Jiaqiang; Mandrus, David G; Pennycook, Stephen J; Pantelides, Sokrates T

2014-06-01

281

New insight into enhanced superconductivity in metals near the metal-insulator transition.  

PubMed

We have studied the transport properties of disordered WSi films near the metal/insulator transition (MIT) and we have also reviewed the data for several other disordered materials near their MIT. In all cases, we found the presence of enhanced superconductivity. We constructed a superconductivity "phase diagram" (i.e., T(c) versus sigma) for each system, which reveals a striking correlation: In all cases, T(c) values are significantly enhanced only for samples whose conductivities lie within a narrow range on the metallic side of, and moderately near, the MIT. We present a heuristic model to explain this phenomenon. PMID:11690447

Osofsky, M S; Soulen, R J; Claassen, J H; Trotter, G; Kim, H; Horwitz, J S

2001-11-01

282

Effects of transition metals on nitric oxide synthase catalysis  

PubMed Central

The biosynthesis of nitric oxide (NO) by the enzyme NO synthase (NOS) proceeds by the hydroxylation of l-arginine to form NG-hydroxy-l-arginine followed by the conversion of NG-hydroxy-l-arginine to l-citrulline and NO. The previously identified requirements of this relatively complicated reaction include several protein-bound cofactors: cytochrome P450-type heme, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and tetrahydrobiopterin (H4B). In addition to l-arginine, NOS also requires the substrates NADPH and molecular oxygen. The role of H4B in NOS catalysis has long been a subject of debate and uncertainty fueled, in part, by the failure to detect any dependence of the NOS reaction on nonheme iron, a cofactor integral to catalysis in every other H4B-dependent enzyme. Here we report the ability of NOS to bind transition metals stoichiometrically, and demonstrate that the rate of catalysis is enhanced by nonheme iron. We also show that other divalent transition metals, including Cu, Zn, Co, and Ni, inhibit NOS catalysis. Also, the addition of Cu2+ to NOS inhibits heme reduction, whereas the addition of Fe2+ does not. Overall, the results appear to connect NOS to the known H4B/nonheme iron-dependent hydroxylases, and suggest a similar, if not identical, step in the NOS reaction mechanism. PMID:9736696

Perry, Jason M.; Marletta, Michael A.

1998-01-01

283

Anderson metal-insulator transitions with classical magnetic impurities  

NASA Astrophysics Data System (ADS)

We study the effects of classical magnetic impurities on the Anderson metal-insulator transition (AMIT) numerically. In particular we find that while a finite concentration of Ising impurities lowers the critical value of the site-diagonal disorder amplitude Wc, in the presence of Heisenberg impurities, Wc is first increased with increasing exchange coupling strength J due to time-reversal symmetry breaking. The resulting scaling with J is compared to analytical predictions by Wegner [1]. The results are obtained numerically, based on a finite-size scaling procedure for the typical density of states [2], which is the geometric average of the local density of states. The latter can efficiently be calculated using the kernel polynomial method [3]. Although still suffering from methodical shortcomings, our method proves to deliver results close to established results for the orthogonal symmetry class [4]. We extend previous approaches [5] by combining the KPM with a finite-size scaling analysis. We also discuss the relevance of our findings for systems like phosphor-doped silicon (Si:P), which are known to exhibit a quantum phase transition from metal to insulator driven by the interplay of both interaction and disorder, accompanied by the presence of a finite concentration of magnetic moments [6].

Jung, Daniel; Kettemann, Stefan

2014-08-01

284

Properties of binary transition-metal arsenides (TAs)  

NASA Astrophysics Data System (ADS)

We present thermodynamic and transport properties of transition-metal (T) arsenides, TAs, with T = Sc to Ni (3d), Zr, Nb, Ru (4d), Hf and Ta (5d). Characterization of these binaries is carried out with powder x-ray diffraction, temperature- and field-dependent magnetization and resistivity, temperature-dependent heat capacity, Seebeck coefficient, and thermal conductivity. All binaries show metallic behavior except TaAs and RuAs. TaAs, NbAs, ScAs and ZrAs are diamagnetic, while CoAs, VAs, TiAs, NiAs and RuAs show approximately Pauli paramagnetic behavior. FeAs and CrAs undergo antiferromagnetic ordering below TN ? 71 K and TN ? 260 K, respectively. MnAs is a ferromagnet below TC ? 317 K and undergoes hexagonal-orthorhombic-hexagonal transitions at TS ? 317 K and 384 K, respectively. For TAs, Seebeck coefficients vary between + 40 and - 40 ?V K-1 in the 2-300 K range, whereas thermal conductivity values stay below 18 W m-1 K-1. The Sommerfeld coefficients ? are less than 10 mJ K-2 mol-1. At room temperature with application of 8 T magnetic field, large positive magnetoresistance is found for TaAs (˜25%), MnAs (˜90%) and NbAs (˜75%).

Saparov, Bayrammurad; Mitchell, Jonathan E.; Sefat, Athena S.

2012-08-01

285

Characteristics of thermal decomposition products of rare earth, alkali earth metal and transition metal p -toluenesulfonates  

Microsoft Academic Search

Summary A series of alkali earth (Ca, Ba), lanthanide (La, Ce, Pr, Nd,) and transition metal (Fe, Mn, Co, Ni, Cu, Zn, Cd) p-toluenesulfonates were synthesized. Thermal analysis, infrared spectrophotometry, and XRD powder diffraction patterns were employed to characterize the final thermal decomposition products. By heating in dynamic air atmosphere at 30-850°C, it has been found that the alkali earth

Z.-L. Sun

2005-01-01

286

XRD and XPS characterisation of transition metal silicide thin films  

NASA Astrophysics Data System (ADS)

Binary transition metal silicides based on the systems Ti-Si, Fe-Si, Ni-Si and Cr-Si were fabricated on Si wafers by means of ion-beam co-sputter deposition and subsequent annealing. The crystalline structures of the phases formed were identified from the characteristic patterns acquired by means of X-ray diffraction (XRD) measurements. The phase formation sequences were described by means of the Pretorius' effective heat of formation (EHF) model. For the Ti-Si, Fe-Si and Ni-Si systems, single phase thin films of TiSi2, ?-FeSi2 and NiSi2 were generated as the model predicts, while a mixture of CrSi + CrSi2 phases was obtained for the Cr-Si system. The surface chemical condition of individual specimens was analysed by using X-ray photoelectron spectroscopy (XPS). The chemical shifts of transition metal 2p3/2 peaks from their metallic to silicide states were depicted by means of the Auger parameters and the Wagner plots. The positive chemical shift of 2.0 eV for Ni 2p3/2 peak of NiSi2 is mainly governed by the initial-state effects. For the other silicide specimens, the initial-state and final-state effects may oppose one another with similar impact. Consequently, smaller binding energy shifts of both negative and positive character are noted; a positive binding energy shift of 0.3 eV for the Fe 2p3/2 level was shown for ?-FeSi2 and negative binding energy shifts of 0.1 and 0.3 eV were determined for CrSi + CrSi2 and TiSi2, respectively.

Tam, P. L.; Cao, Y.; Nyborg, L.

2012-02-01

287

Angle resolved photoemission studies of transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

High resolution angle resolved photoelectron spectroscopy was used to study the electronic structures of several transition metal dichalcogenides. These materials are of interest as quasi-two-dimensional layered materials which undergo charge density wave (CDW) phase transitions. Five specific transition metal dichalcogenides were studied: 2H-TaSe2, 2H-TaS2, 2H-NbSe2, 1T-TaS2, and 1T-TiSe2. The band dispersions and Fermi surfaces of each material were investigated. Results were compared to those of band structure calculations. Comparing spectra taken in the high temperature states and the lower temperature CDW state, information about the CDW energy gaps and the CDW mechanisms were obtained. For the 2H polytype materials, the normal state electronic structures have the same basic characteristics. Their Fermi surface consists of a pocket around Gamma and another one around K. They all exhibit a saddle band near EF along Gamma-K. In the CDW phases, a finite energy gap is observed in the saddle band region and on the Fermi surface around K in 2H-TaS 2 and 2H-TaSe2. Gapping on the Fermi surface around Gamma is not observed. Thus, the charge density waves in this family of materials originate from a combination of the partial nesting of the Fermi surface around K and the saddle band. For 1T-TaS2, a Fermi surface was observed in the quasi-commensurate CDW phase. In addition to the pockets around the M points as predicted by earlier non-relativistic non-self-consistent band structure calculations for the normal state, a small electron pocket around Gamma was observed. The latter pocket is consistent with results of recent self-consistent band structure calculations including spin-orbit interaction. In the commensurate CDW phase, a k-independent energy gap of approximately 120 meV magnitude opens at the Fermi level as a result of a CDW-induced Mott transition. 1T-TiSe2 shows significant three-dimensional electronic structure despite its layered morphology. It also shows interesting photoelectron spectroscopy effects, such as emission from thermally populated states above the Fermi level. The CDW formation seems to result from coupling of hole and electron states in an excitonic insulator transition.

Tonjes, Wayne C.

288

Fermi surface nesting in several transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2, the nesting vector is present despite different doping levels, which leads us to expect a possible enhancement of the CDW instability with Cu intercalation in the CuxNbS2 family of materials.

Inosov, D. S.; Zabolotnyy, V. B.; Evtushinsky, D. V.; Kordyuk, A. A.; Büchner, B.; Follath, R.; Berger, H.; Borisenko, S. V.

2008-12-01

289

Structures of late transition metal monoxides from Jahn-Teller instabilities in the rock salt lattice.  

PubMed

Most late transition metal (LTM) monoxides crystallize in other than a rock salt structure, which is so common in the earlier transition metal monoxides. Here we present theoretical evidence based on density functional theory that an electron-phonon coupling involving a single soft mode in the cubic cell is responsible for the onset of the experimentally observed structures of the late transition metal monoxides. PMID:25062207

Derzsi, Mariana; Piekarz, Przemys?aw; Grochala, Wojciech

2014-07-11

290

Transition metal-catalyzed oxidation of sulfur(IV) oxides. Atmospheric-relevant processes and mechanisms  

Microsoft Academic Search

The transition metal-catalyzed oxidation of sulfur(IV) oxides has been known for more than 100 years. There is a significant lack of information on the actual role of the transition metal-catalyzed reactions, and much of the earlier work was performed without a detailed knowledge of the chemical system. For this reason attention is focused on the role of transition metal ions

Christian. Brandt; Rudi. van Eldik

1995-01-01

291

Insulator-to-metal transition in vanadium sesquioxide: does the Mott criterion work in this case?  

Microsoft Academic Search

It is shown that the Mott criterion expressed by the simple relation aB(nc)???0.25 turns out to be quite successful in describing metal–insulator phase transitions not only in heavily doped semiconductors, but also in transition metal oxides such as VO2 and V2O3. It is found in this article that, in the case of a high-temperature transition ‘paramagnetic insulator – paramagnetic metal

Alexander Pergament; Genrikh Stefanovich

2011-01-01

292

Insulator-to-metal transition in vanadium sesquioxide: does the Mott criterion work in this case?  

Microsoft Academic Search

It is shown that the Mott criterion expressed by the simple relation aB(nc)???0.25 turns out to be quite successful in describing metal–insulator phase transitions not only in heavily doped semiconductors, but also in transition metal oxides such as VO2 and V2O3. It is found in this article that, in the case of a high-temperature transition ‘paramagnetic insulator – paramagnetic metal

Alexander Pergament; Genrikh Stefanovich

2012-01-01

293

Solution synthesis of nanoparticular binary transition metal antimonides.  

PubMed

The preparation of nanoengineered materials with controlled nanostructures, for example, with an anisotropic phase segregated structure or a regular periodicity rather than with a broad range of interparticle distances, has remained a synthetic challenge for intermetallics. Artificially structured materials, including multilayers, amorphous alloys, quasicrystals, metastable crystalline alloys, or granular metals, are mostly prepared using physical gas phase procedures. We report a novel, powerful solution-mediated approach for the formation of nanoparticular binary antimonides based on presynthesized antimony nanoparticles. The transition metal antimonides M-Sb (M = Co, Ni, Cu(2), Zn) were obtained with sizes ranging from 20 and 60 nm. Through careful control of the reaction conditions, single-phase nanoparticular antimonides were synthesized. The nanophases were investigated by powder X-ray diffraction and (high resolution) electron microscopy. The approach is based on activated metal nanoparticles as precursors for the synthesis of the intermetallic compounds. X-ray powder diffraction studies of reaction intermediates allowed monitoring of the reaction kinetics. The small particle size of the reactants ensures short diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. PMID:21736318

Kieslich, Gregor; Birkel, Christina S; Stewart, Andrew; Kolb, Ute; Tremel, Wolfgang

2011-08-01

294

Early transitional metal alkyl, alkylidene, and alkylidyne chemistry  

E-print Network

CHAPTER 1. Zirconium and hafnium complexes of several new unsymmetric diamide ligands have been prepared and their proficiency in olefin polymerization reactions evaluated. The first set of supporting ligands examined are ...

Tonzetich, Zachary John

2007-01-01

295

Time-dependent Laudau theory of charge-density waves in transition-metal dichalcogenides  

Microsoft Academic Search

A time-dependent Landau theory is proposed for the transition-metal dichalcogenides coupling the motion of the charge-density waves to the electron liquid. The dissipative terms are related to the metallic resistance. The \\

W. L. McMillan

1975-01-01

296

Electrochemical lithiation and delithiation for control of magnetic properties of nanoscale transition metal oxides  

E-print Network

Transition metal oxides comprise a fascinating class of materials displaying a variety of magnetic and electronic properties, ranging from half-metallic ferromagnets like CrO2, ferrimagnetic semiconductors like Fey's, and ...

Sivakumar, Vikram

2008-01-01

297

Deviance as an antecedent and consequence of early transitions to adulthood: mediating effects and moderating conditions  

E-print Network

was estimated using the full sample (N= 3,379) to examine direct associations among adolescent deviance, early transitions to adulthood, and adult deviance while controlling for prior involvement in deviant behavior in adolescence. An expanded seven latent...

Halim, Shaheen

2005-08-29

298

Transition Metal Chalcogenides: Ultrathin Inorganic Materials with Tunable Electronic Properties.  

PubMed

Conspectus After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ?1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ?10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer. PMID:25489917

Heine, Thomas

2014-12-01

299

Dense organic-inorganic framework materials containing transition metal ions  

NASA Astrophysics Data System (ADS)

Hybrid inorganic-organic framework materials built upon metal cations and polyfunctional anionic ligands have arrived at the forefront of chemical research, boasting unique properties that derive from their diverse structures. The majority of these compounds are prepared through mild hydrothermal or solvothermal synthesis, at temperatures below 225°C. There has been a great deal of emphasis on porous coordination polymers, or "metal-organic frameworks" (MOFs), which exhibit a wide range of useful sorption and catalytic properties, but there is growing interest in a second class of hybrid frameworks which are denser and often have extended inorganic connectivity, which may be thought of as hybrid metal oxides. These denser compounds provide access to more oxide-like properties such as magnetism, along with improved thermal stability. The flourishing diversity of structures and dimensionalities seen in these materials has led to their growth into a major field of research. Here, the hydrothermal syntheses and crystal structures of fourteen new hybrid framework materials are reported, and additional properties are measured for several of them. The terminology used to describe this class of materials is laid out, and a classification scheme is introduced based on the dimensionalities of their structures. While a combinatorial approach for new compound discovery is often used, strategies for rational synthesis are also discussed. A family of isostructural 3,4,5-trihydroxybenzoate (gallate) hybrid frameworks of the first-row transition metals is produced, with chiral structures imparted by helical packing arrangements. Complex magnetic behavior is characterized in a pillared layered cobalt ethanedisulfonate, the structure of which features intricate metal-oxygen-metal connectivity. Frameworks of new, sometimes unreported, ligand molecules are created through the reactivities of aspartic acid and 5-hydroxyisophthalic acid under hydrothermal conditions. A family of two-ligand frameworks are created using 5-hydroxyisophthalic acid and 4,4'-bipyridyl, which form very different structures when reacted with Mn, Ni, Cu and Zn. Finally, reactions of Cu and Zn together with these two ligands lead to a family of four- and five-component hybrid frameworks showing even greater degrees of structural complexity.

Feller, Russell Kenneth

2008-10-01

300

Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates  

SciTech Connect

Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The functional groups on the carbon nanofiber templates were essential for the coating of these templates: they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the combustion reaction between the carbon nanofibers and oxygen. - Graphical abstract: Mono and binary transition metal-oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air.

Ogihara, Hitoshi, E-mail: ogihara@cms.titech.ac.j [Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002 (Japan); Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo 102-8472 (Japan); Masahiro, Sadakane [Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002 (Japan); Nodasaka, Yoshinobu [Oral Functional Science, Graduate School of Dental Medicine, Hokkaido University, N13-W7 Kita-ku, Sapporo 060-8586 (Japan); Ueda, Wataru, E-mail: ueda@cat.hokudai.ac.j [Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002 (Japan)

2009-06-15

301

Surfactant-Modified Diffusion on Transition-Metal Surfaces  

SciTech Connect

Wanting to convert surface impurities from a nuisance to a systematically applicable nano-fabrication tool, we have sought to understand how such impurities affect self-diffusion on transition-metal surfaces. Our field-ion microscope experiments reveal that in the presence of surface hydrogen, self-diffusion on Rh(100) is promoted, while on Pt(100), not only is it inhibited, but its mechanism changes. First-principles calculations aimed at learning how oxygen fosters perfect layerwise growth on a growing Pt(111) crystal contradict the idea in the literature that it does so by directly promoting transport over Pt island boundaries. The discovery that its real effect is to burn off adventitious adsorbed carbon monoxide demonstrates the predictive value of state-of-the-art calculation methods.

FEIBELMAN,PETER J.; KELLOGG,GARY LEE

1999-12-01

302

Spin susceptibility of two-dimensional transition-metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We have obtained analytical expressions for the q-dependent static spin susceptibility of monolayer transition-metal dichalcogenides, considering both the electron-doped and hole-doped cases. Our results are applied to calculate spin-related physical observables of monolayer MoS2, focusing especially on in-plane/out-of-plane anisotropies. We find that the hole-mediated Ruderman-Kittel-Kasuya-Yosida exchange interaction for in-plane impurity-spin components decays with the power law R-5/2 as a function of distance R, which deviates from the R-2 power law normally exhibited by a two-dimensional Fermi liquid. In contrast, the out-of-plane spin response shows the familiar R-2 long-range behavior. We also use the spin susceptibility to define a collective g factor for hole-doped MoS2 systems and discuss its density-dependent anisotropy.

Hatami, H.; Kernreiter, T.; Zülicke, U.

2014-07-01

303

Transition Metal Dichalcogenide Growth via Close Proximity Precursor Supply.  

PubMed

Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components. PMID:25487822

O'Brien, Maria; McEvoy, Niall; Hallam, Toby; Kim, Hye-Young; Berner, Nina C; Hanlon, Damien; Lee, Kangho; Coleman, Jonathan N; Duesberg, Georg S

2014-01-01

304

A novel method of preparing transition metal nanoparticles.  

PubMed

The hydrolysis behaviour of some Mg-rich compounds (Mg2Ni, Mg2Cu, Mg54Ag17 and Mg3Au) at room temperature has been systematically investigated. Ni, Cu, Ag and Au nanoparticles have successfully been prepared by the hydrolysis of these compounds. The Ni, Cu, Ag, and Au nanoparticles obtained by the hydrolysis method are close to spherical in shape with a particle size below 20 nm. A continuous skeleton of magnesium atoms could be the channel for the dissolution of Mg atoms in these Mg-rich compounds, which is helpful in the hydrolysis of these Mg-rich compounds. The hydrolysis of the Mg-rich compounds provides a new and relatively simple method for the production of transition metal nanoparticles. PMID:19441544

Wang, Huabin; Northwood, Derek O

2009-02-01

305

Spin and pseudospins in layered transition metal dichalcogenides  

SciTech Connect

The recent emergence of two-dimensional layered materials in particular the transition metal dichalcogenides provides a new laboratory for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. These degrees of freedom are the real electron spin, the layer pseudospin, and the valley pseudospin. New methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties and strong spin orbit coupling. The former leads to the versatile control of the valley pseudospin, whereas the latter gives rise to an interplay between the spin and the pseudospins. Here, we provide a brief review of both theoretical and experimental advances in this field.

Xu, Xiaodong [University of Washington] [University of Washington; Yao, Wang [University of Hong Kong, The] [University of Hong Kong, The; Xiao, Di [Carnegie Mellon University (CMU)] [Carnegie Mellon University (CMU); Heinz, Tony F. [Columbia University, New York] [Columbia University, New York

2014-01-01

306

Transition Metal Dichalcogenide Growth via Close Proximity Precursor Supply  

PubMed Central

Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components. PMID:25487822

O'Brien, Maria; McEvoy, Niall; Hallam, Toby; Kim, Hye-Young; Berner, Nina C.; Hanlon, Damien; Lee, Kangho; Coleman, Jonathan N.; Duesberg, Georg S.

2014-01-01

307

Continuously tunable electronic structure of transition metal dichalcogenides superlattices  

PubMed Central

Two dimensional transition metal dichalcogenides have very exciting properties for optoelectronic applications. In this work we theoretically investigate and predict that superlattices comprised of MoS2 and WSe2 multilayers possess continuously tunable electronic structure with direct bandgaps. The tunability is controlled by the thickness ratio of MoS2 versus WSe2 of the superlattice. When this ratio goes from 1:2 to 5:1, the dominant K-K direct bandgap is continuously tuned from 0.14?eV to 0.5?eV. The gap stays direct against ?0.6% to 2% in-layer strain and up to ?4.3% normal-layer compressive strain. The valance and conduction bands are spatially separated. These robust properties suggest that MoS2 and WSe2 multilayer superlattice should be a promising material for infrared optoelectronics. PMID:25677917

Zhao, Yong-Hong; Yang, Feng; Wang, Jian; Guo, Hong; Ji, Wei

2015-01-01

308

Transition Metal Dichalcogenide Growth via Close Proximity Precursor Supply  

NASA Astrophysics Data System (ADS)

Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components.

O'Brien, Maria; McEvoy, Niall; Hallam, Toby; Kim, Hye-Young; Berner, Nina C.; Hanlon, Damien; Lee, Kangho; Coleman, Jonathan N.; Duesberg, Georg S.

2014-12-01

309

Moderate temperature sodium cells. I - Transition metal disulfide cathodes  

NASA Technical Reports Server (NTRS)

TiS2, VS2, and Nb(1.1)S2 transition metal disulfides were evaluated as cathode materials for a moderate temperature rechargeable Na cell operating at 130 C. The 1st discharge of TiS2 results in a capacity of 0.85 eq/mole; approximately half of the Na in the 1st phase spanning the Na range from zero to 0.30 and almost all the Na in the 2nd phase spanning the 0.37 to 0.80 range are rechargeable. VS2 intercalates up to one mole of Na/mole of VS2 in the 1st discharge; the resulting Na(x)VS2 ternary consists of 3 phases in the 3 ranges of Na from zero to 1. Niobium disulfide undergoes a phase change in the 1st discharge; the average rechargeable capacity in extended cycling of this cathode is 0.50 eq/mole.

Abraham, K. M.; Pitts, L.; Schiff, R.

1980-01-01

310

Hubbard Thomas Fermi theory of transition metal oxide heterostructures  

NASA Astrophysics Data System (ADS)

We demonstrate that the charge distributions in Hubbard-model representations of transition metal oxide heterojunctions can be described by a Thomas-Fermi theory in which the energy is approximated as the sum of the electrostatic energy and the uniform three-dimensional Hubbard model energy per site at the local density equals to a constant. When charged atomic layers in the oxides are approximated as two-dimensional sheets with uniform charge density, the electrostatic energy is simply evaluated. We find that this Thomas-Fermi theory can reproduce results obtained from full Hartree-Fock theory for various different heterostructures. We also show explicitly how Thomas-Fermi theory can be used to estimate some key properties qualitatively.

Lee, Wei-Cheng; MacDonald, A. H.

2008-04-01

311

Transition metal surfaces under oxygen-rich conditions (abstract only)  

NASA Astrophysics Data System (ADS)

The catalytic oxidation activity of transition metal particles in automobile catalysts is influenced by the superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of ultrathin oxide films on low-index surfaces of transition metals by means of first-principles atomistic thermodynamics calculations based on density functional theory. On Pd(111) a surface oxide with Pd5O4 stoichiometry is stable, which does not correspond to any bulk structure. At the same time, a variety of metastable structures form. On Pd(100) a PdO(101) trilayer is the stable surface oxide. All structures display two-oxygen-coordinated and four-oxygen-coordinated Pd atoms, the difference lying in the spatial arrangement of these basic structural elements. On rhodium surfaces a Rh oxide trilayer forms that corresponds to a bulk oxide structure. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of ?-PtO2, which appears not to be reactive to either methane dissociation or carbon monoxide adsorption. On Pt(100) an ?-PtO2 layer is thermodynamically the most stable, even if Pt3O4 films could grow in a coherent and stress-free manner. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed in molecular dynamics simulations to occur without any appreciable energy barrier.

Seriani, Nicola

2008-02-01

312

A new model for magnetism in transition metals  

NASA Astrophysics Data System (ADS)

Traditionally, quantum theories of magnetism use either a localized or an itinerant picture of the electrons as a starting point. However, the magnetic properties of transition metals cannot be fully described by either one picture or the other and the behavior of the electrons falls somewhere between these two extremes. Contemporary theories for transition metals attempt to amalgamate these two very different: pictures of electronic behavior. This thesis describes a lattice model which attempts to combine the localized and itinerant pictures in a novel way. The Hamiltonian for this model consists of two pieces. One term describes the electrostatic interaction between two d-electrons which are located on the same atomic site. These interactions become increasingly important as the electrons become more localized. The other term allows for the hopping of electrons from one atomic site to another. The linear combination of atomic orbitals (LCAO) method is used to generate this term which incorporates the physics of itinerant electrons into the model. Calculations are made for one and two-dimensional lattices using two different numerical techniques. The tendency towards broken symmetry states in our model will first be explored using a Hartree-Fock approximation. The approximate treatment of the interactions raises some questions about the validity of the Hartree-Fock approach in these types of calculations. Consequently, quantum Monte Carlo (QMC) calculations, which are formally exact, are performed as well. In practice, statistical errors in QMC calculations become large at low temperatures for fermionic systems. Our preliminary studies show that this problem is much more serious for our model than it is for the less realistic Hubbard model, which is studied extensively in the literature.

Banerjee, Sunoy Neil

2000-10-01

313

Infusing Early Intervention for Substance Use Into Community Mental Health Services for Transitioning Youth  

Microsoft Academic Search

The treatment of transitioning youth (ages 16–25) diagnosed with co-occurring mental health and substance abuse problems has been the focus of numerous research and practice efforts. However, far less attention has been devoted to intervening early to address alcohol and other drug (AOD) use, which has the potential to contribute to the numerous challenges transitioning youth with emotional or behavioral

Sarah A. Taylor; Elizabeth K. Anthony

2011-01-01

314

Metal-Semiconductor Barrier Modulation for High Photoresponse in Transition Metal Dichalcogenide Field Effect Transistors  

PubMed Central

A gate-controlled metal-semiconductor barrier modulation and its effect on carrier transport were investigated in two-dimensional (2D) transition metal dichalcogenide (TMDC) field effect transistors (FETs). A strong photoresponse was observed in both unipolar MoS2 and ambipolar WSe2 FETs (i) at the high drain voltage due to a high electric field along the channel for separating photo-excited charge carriers and (ii) at the certain gate voltage due to the optimized barriers for the collection of photo-excited charge carriers at metal contacts. The effective barrier height between Ti/Au and TMDCs was estimated by a low temperature measurement. An ohmic contact behavior and drain-induced barrier lowering (DIBL) were clearly observed in MoS2 FET. In contrast, a Schottky-to-ohmic contact transition was observed in WSe2 FET as the gate voltage increases, due to the change of majority carrier transport from holes to electrons. The gate-dependent barrier modulation effectively controls the carrier transport, demonstrating its great potential in 2D TMDCs for electronic and optoelectronic applications. PMID:24509565

Li, Hua-Min; Lee, Dae-Yeong; Choi, Min Sup; Qu, Deshun; Liu, Xiaochi; Ra, Chang-Ho; Yoo, Won Jong

2014-01-01

315

Virtual vs. Physical Materials in Early Science Instruction: Transitioning to an Autonomous  

E-print Network

Virtual vs. Physical Materials in Early Science Instruction: Transitioning to an Autonomous Tutor and effects of virtual materials vs. physical materials in early science instruction. In this paper we first lay out a framework for comparing key aspects of this virtual-physical issue, and then we describe

Klahr, David

316

Strong Periodic Lattice Distortion in Transition Metal Dichalcogenides  

NASA Astrophysics Data System (ADS)

The charge density wave (CDW) instability was initially proposed to be the result of the Peierls mechanism in which a divergence in electronic response function results in a periodic charge redistribution; i.e. the electron gas itself is unstable with respect to the formation of a periodically varying electron charge density. However, the mechanism of CDW in many 2D Transition Metal Dichalcogenide (TMD) is still under debate. Fermi surface nesting was originally believed to act as the driving mechanism of CDW transitions in these materials; however, recent reports from both theoretical and experimental studies are not quite within this simple model. We use Spectroscopic Imaging Scanning Tunneling Microscope (SI-STM) to study the surfaces of 2H-TaSe2, 2H-TaS2, and 2H-NbSe2 at various temperatures from 6K to above 100K. Topographic images and differential conductance data were recorded and analyzed in order to help understanding the underlying physics of CDW phases. Our results shows that Periodic Lattice Distortion (PLD) likely plays a more important role than the charge modulation in 2D TMD.

Dai, Jixia; Calleja, Eduardo; Cao, Yue; Dessau, Daniel; McElroy, Kyle; Berger, Helmuth; Zhu, Xiangde; Li, Lijun; Sun, Yuping; Wolf, T.

2012-02-01

317

Antiferromagnetic metal and Mott transition on Shastry-Sutherland lattice.  

PubMed

The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems. PMID:24777282

Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

2014-01-01

318

Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice  

PubMed Central

The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems. PMID:24777282

Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

2014-01-01

319

Metal-insulator transition in dirty Kondo insulators  

SciTech Connect

We consider arbitrary clusters of Kondo holes in a Kondo insulator described by the nondegenerate symmetric Anderson lattice with a nearest-neighbor tight-binding conduction band on a simple cubic lattice. The {ital f}-electron self energy is considered within the local approximation. Each Kondo hole introduces a boundstate in the gap. The quantum interference in the scattering off the impurities gives rise to interactions among the Kondo holes. The spectral weight of the bound states is predominantly localized on the sites neighboring the Kondo holes. Clusters of impurities separated by more than one lattice site are disconnected for boundstates at the Fermi level. On a simple cubic lattice the metal-insulator transition in the impurity band then reduces to the site percolation of Kondo holes with first, second and fourth nearest neighbors. We use the low density mean cluster size expansion and a small cell renormalization to estimate the critical concentration. Hopping in the conduction band beyond nearest neighbors reduces the percolation threshold. Hence, 9.9{percent} of Kondo holes is an upper bound for the insulator to become a metal. {copyright} {ital 1996 American Institute of Physics.}

Schlottmann, P.; Hellberg, C.S. [Department of Physics, Florida State University, Tallahassee, Florida 32306 (United States)] [Department of Physics, Florida State University, Tallahassee, Florida 32306 (United States)

1996-04-01

320

Watching Transitions Unfold: A Mixed-Method Study of Transitions within Early Childhood Care and Education Settings  

ERIC Educational Resources Information Center

Unlike the transitions children make between settings, those they undertake between age groups within early childhood care and education (ECCE) settings are seldom studied. Accordingly, this exploratory study followed seven preschool children (three boys and four girls) as they moved to new rooms in five ECCE settings. Structured observations of…

O'Farrelly, Christine; Hennessy, Eilis

2014-01-01

321

Chemical synthetic strategy for single-layer transition-metal chalcogenides.  

PubMed

A solution-phase synthetic protocol to form two-dimensional (2D) single-layer transition-metal chalcogenides (TMCs) has long been sought; however, such efforts have been plagued with the spontaneous formation of multilayer sheets. In this study, we discovered a solution-phase synthetic protocol, called "diluted chalcogen continuous influx (DCCI)", where controlling the chalcogen source influx (e.g., H2S) during its reaction with the transition-metal halide precursor is the critical parameter for the formation of single-layer sheets as examined for the cases of group IV TMCs. The continuous influx of dilute H2S throughout the entire growth period is necessary for large sheet formation through the exclusive a- and b-axial growth processes. By contrast, the burst influx of highly concentrated H2S in the early stages of the growth process forms multilayer TMC nanodiscs. Our DCCI protocol is a new synthetic concept for single-layer TMCs and, in principle, can be operative for wide range of TMC nanosheets. PMID:25313652

Yoo, Dongwon; Kim, Minkyoung; Jeong, Sohee; Han, Jeonghee; Cheon, Jinwoo

2014-10-22

322

Solubility Behavior and Phase Stability of Transition Metal Oxides in Alkaline Hydrothermal Environments  

SciTech Connect

The solubility behavior of transition metal oxides in high temperature water is interpreted by recognizing three types of chemical reaction equilibria: metal oxide hydration/dehydration, metal oxide dissolution and metal ion hydroxocomplex formation. The equilibria are quantified using thermodynamic concepts and the thermochemical properties of the metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., element shaving atomic numbers between Z = 22 (Ti) and Z = 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted in the laboratory. Particular attention is devoted to the uncharged metal ion hydrocomplex, M{sup Z}(OH){sub Z}(aq), since its thermochemical properties define minimum solubilities of the metal oxide at a given temperature. Experimentally-extracted values of standard partial molal entropy (S{sup 0}) for the transition metal ion neutral hydroxocomplex are shown to be influenced by ligand field stabilization energies and complex symmetry.

S.E. Ziemniak

2000-05-18

323

A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis  

Microsoft Academic Search

A literature review of modern transition-metal nanoclusters, with an emphasis on those nanoclusters which are catalytically active, is presented in two parts. Part One presents background information on transition-metal nanoclusters, including an overview of common synthetic routes, a description of how nanoclusters are stabilized, and a brief summary of the multiple characterization techniques used (and the type of information that

John D. Aiken; Richard G. Finke

1999-01-01

324

Transition-metal nanocluster stabilization for catalysis: A critical review of ranking methods and putative stabilizers  

Microsoft Academic Search

A significant problem in the burgeoning transition-metal nanocluster literature is the myriad of proposed, putative stabilizers. A main objective of the present contribution is to provide a critical review of the methods for, and current rankings of, claimed transition-metal nanocluster stabilizers, with a focus on catalytically active nanoclusters. Following a brief introduction to the literature methods for evaluating colloidal stabilizers

Lisa Starkey Ott; Richard G. Finke

2007-01-01

325

Endogenous transitional metal and proximate analysis of selected medicinal plants from Pakistan  

Microsoft Academic Search

Transitional metal and proximate analysis plays an important role in assessing the appropriateness of medicinal plants or their extracts orally taken by the marginal communities. To acquaint, various proximate parameters like ash, carbohydrate, protein, fiber, fat and moisture (both dry and wet) and endogenous transition metals like copper, magnesium, manganese, lead, cadmium, iron, chromium and sodium; four medicinal plant species,

Javid Hussain; Riaz Ullah; Najeeb ur Rehman; Abdul Latif Khan; Zia Muhammad; Farman Ullah Khan; Syed Tasleem Hussain; Saeed Anwar

326

The substructure of deformed carbides of the transition metals in their homogeneity regions  

Microsoft Academic Search

Carbides of the transition metals possess a number of valuable properties for some present-day engineering applications, such as high refractoriness and excellent elasticity, hardness, and wear resistance characteristics. At the same time, however, use of these compounds is largely restricted by their low ductility. All carbides of the transition metals of groups IV-V have fairly wide homogeneity regions, within which

G. V. Samsonov; I. I. Timofeeva; L. A. Klochkov

1969-01-01

327

XAFS Studies of Transition Metal and Halogen Biomaterials in Invertebrate Tools  

E-print Network

XAFS Studies of Transition Metal and Halogen Biomaterials in Invertebrate Tools Y. Taoad , J, invertebrate tools, biomaterials PACS: 87.64.Fb INTRODUCTION There exists a large amount of transition metals and halide elements in many invertebrate tools [1]. What is the biological role of these elements

Scott, Robert A.

328

Evidence of Transition-Metal Accumulation on Aged Graphite Anodes by SIMS  

E-print Network

Evidence of Transition-Metal Accumulation on Aged Graphite Anodes by SIMS D. P. Abraham,a, *,z T-based positive and graphite-based negative electrodes, a significant portion of cell impedance rise, and aged graphite electrodes. The data clearly indicate that these transition-metal elements accumulate

Spila, Timothy P.

329

ON THE PRESSURE DEPENDENCE OF SUPERCONDUCTIVITY IN TRANSITION METAL DICHALCOGENIDE LAYER CRYSTALS  

E-print Network

L-279 ON THE PRESSURE DEPENDENCE OF SUPERCONDUCTIVITY IN TRANSITION METAL DICHALCOGENIDE LAYER measurements of the pressure dependence of superconductivity in transition metal dichalco- genide layer, the critical tempe- rature 7c and the critical field Hc2, show very little variation with pressure, at least up

Boyer, Edmond

330

Synthesis, Characterization, and Biological Activity of Transition Metal Complexes of Oxadiazole  

Microsoft Academic Search

A new oxadiazole and its metal complexes have been synthesized and characterized. The ligand potassium 2-(2-phenylisonicotinoyl)hydrazinecarbodithioate [K(H2L)] (1) on reaction with transition metals in EtOH-H2O medium undergoes cyclization and converted to 5-(2-phenyl-4-pyridyl)-1,3,4-oxadiazole-2-thione (ppot), which yielded a series of mononuclear transition metal complexes. Four transition metal complexes of ligand are prepared and characterized based on elemental analysis, IR,H NMR, UV-vis spectra,

D Harikishore Kumar Reddy; Y Harinath; Y Suneetha; K Seshaiah; A V. R. Reddy

2011-01-01

331

Fermi surfaces and electronic topological transitions in metallic solid solutions  

NASA Astrophysics Data System (ADS)

Notwithstanding the substitutional disorder, the Fermi surface of metallic alloys can be measured and computed. We show that, from the theoretical point of view, it is defined as the locus of the peaks of the Bloch Spectral Function (BSF). Such Fermi surfaces, on varying the atomic concentrations, may undergo changes of their topology, known as Electronic Topological Transitions (ETT). Thus, for instance, pockets of electrons or holes may appear or disappear, necks may open or close. ETTs cause anomalous behaviours of thermodynamic, transport and elastic properties of metals and constitute a fascinating field in the study of Fermi liquid systems. Although ETTs could be studied on pure systems as a function of the thermodynamic variables, nevertheless such a study would often require extreme conditions, and would lead to experimental difficulties. On the other hand, it is possible to explore the variations of atomic concentration, i.e. the valence electron per atom ratio, in metallic solid solutions with a relative experimental ease. In this paper we review the theoretical techniques for the determination of Fermi surfaces in metallic solid solutions and discuss some examples of ETTs, namely Li?Mg, Zr?Nb, Nb?Mo, Mo?Re, Ag?Pd, Cd?Mg, Ni?W and Ni?Ti alloys, also in connection with experimental data as thermoelectric power, resistivity, elastic constants and electron-phonon coupling and with the determinations of the electron momentum distribution function from Compton scattering and positron annihilation experiments. We show that the ab initio calculations of the electronic structure for the quoted systems, together with a careful determination of the BSF, are able to predict quantitatively ETTs at those concentrations where physical quantities display anomalies, so confirming directly ETT theory. Although it is not the purpose of the present review to give a full account of electronic structure calculation schemes, however, we briefly discuss the ideas and the main physical approximations underlying theories of substitutional disorder in alloys. We shall pay some more attention to the Coherent Potential Approximation (CPA) in the Korringa-Kohn-Rostoker (KKR) multiple scattering framework and the Hohenberg and Kohn Density Functional Theory in the Local Density Approximation (LDA) for the exchange-correlation potential. The above choice is supported by the numerical versatility of the LDA?KKR?CPA theory, and, more important, by the a fortiori evidence that essentially equivalent results are obtained from different theoretical frameworks, provided the same basic physical approximations are used. Accordingly, when convenient, we present new LDA?KKR?CPA determinations of the Fermi surfaces, as for the Zr?Nb?Mo?Re series.

Bruno, E.; Ginatempo, B.; Guiliano, E. S.; Ruban, A. V.; Vekilov, Yu. Kh.

1994-12-01

332

Dissolution energetics and its strain dependence of transition metal alloying elements in tungsten  

NASA Astrophysics Data System (ADS)

We have systematically investigated the dissolution energetics of all 3d-5d transition metal alloying elements (AEs) in tungsten (W) using a first-principles method. It is found that Ti is the easiest one to dissolve with the solution energy of -0.81 eV in the intrinsic bulk W, while La is the most difficult one with the solution energy as large as 3.91 eV. The d-electrons play a determining role in the dissolution of AEs in W. We have demonstrated that the solution energies of AEs is a linear monotonic function of strain. The binding energy results indicate that the interaction between AEs is repulsive for the early elements and then becomes attractive for the late elements in each series. This study provides a good reference for developing W materials as a plasma facing material.

Yan, Wen-Li; Zhou, Hong-Bo; Jin, Shuo; Zhang, Ying; Lu, Guang-Hong

2015-01-01

333

The Early College Challenge: Navigating Disadvantaged Students' Transition to College  

ERIC Educational Resources Information Center

Successful early college high schools (ECHSs) are formed through partnerships between high schools and colleges (usually community colleges). Think of it as preparation through acceleration. ECHSs enroll disadvantaged students who have not excelled with ordinary grade-level academic content and have them take college courses while still in high…

Rosenbaum, James E.; Becker, Kelly Iwanaga

2011-01-01

334

Methods for Detecting Early Warnings of Critical Transitions in Time Series Illustrated Using Simulated Ecological Data  

PubMed Central

Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called ‘early warning signals’, and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data. PMID:22815897

Dakos, Vasilis; Carpenter, Stephen R.; Brock, William A.; Ellison, Aaron M.; Guttal, Vishwesha; Ives, Anthony R.; Kéfi, Sonia; Livina, Valerie; Seekell, David A.; van Nes, Egbert H.; Scheffer, Marten

2012-01-01

335

Discovering the electronic circuit diagram of life: structural relationships among transition metal binding sites in oxidoreductases  

PubMed Central

Oxidoreductases play a central role in catalysing enzymatic electron-transfer reactions across the tree of life. To first order, the equilibrium thermodynamic properties of these proteins are governed by protein folds associated with specific transition metals and ligands at the active site. A global analysis of holoenzyme structures and functions suggests that there are fewer than approximately 500 fundamental oxidoreductases, which can be further clustered into 35 unique groups. These catalysts evolved in prokaryotes early in the Earth's history and are largely responsible for the emergence of non-equilibrium biogeochemical cycles on the planet's surface. Although the evolutionary history of the amino acid sequences in the oxidoreductases is very difficult to reconstruct due to gene duplication and horizontal gene transfer, the evolution of the folds in the catalytic sites can potentially be used to infer the history of these enzymes. Using a novel, yet simple analysis of the secondary structures associated with the ligands in oxidoreductases, we developed a structural phylogeny of these enzymes. The results of this ‘composome’ analysis suggest an early split from a basal set of a small group of proteins dominated by loop structures into two families of oxidoreductases, one dominated by ?-helices and the second by ?-sheets. The structural evolutionary patterns in both clades trace redox gradients and increased hydrogen bond energy in the active sites. The overall pattern suggests that the evolution of the oxidoreductases led to decreased entropy in the transition metal folds over approximately 2.5 billion years, allowing the enzymes to use increasingly oxidized substrates with high specificity. PMID:23754810

Kim, J. Dongun; Senn, Stefan; Harel, Arye; Jelen, Benjamin I.; Falkowski, Paul G.

2013-01-01

336

Metal-Insulator Transition in Dilute 2D Electron and Hole Gases  

Microsoft Academic Search

\\u000a The existence of a critical metal-insulator transition in two-dimensional (2D) systems has been a subject of great debate\\u000a over the last few decades. The scaling theory of electron localisation [1] predicts a critical metal-to-insulator transition with increasing disorder only in 3D, and no such transition in 2D. According\\u000a to this theory, the resistance of a 2D sample should monotonically increase

A. K. Savchenko

337

Pyridinyl hydrazone derivatives of thiacalix[4]arene as selective extractants of transition metal ions  

Microsoft Academic Search

The recognition ability of pyridinyl hydrazone derivatives of cone- and 1,3-alternate tetrathiacalix[4]arenes towards transition and alkali metals has been investigated by picrate extraction method. The stoichiometry\\u000a of complexes and the extraction constants have been determined. It has been found that hydrazones do not extract alkali metal\\u000a ions but show an excellent affinity towards transition and heavy metal cations. The removal

Sergey N. PodyachevNadezda; Nadezda E. Burmakina; Victor V. Syakaev; Svetlana N. Sudakova; Wolf D. Habicher; Alexander I. Konovalov

338

Aneuploidy as an Early Mechanistic Event in Metal Carcinogenesis  

PubMed Central

Aneuploidy has recently been proposed as an initiating event for carcinogenesis. There is significant evidence that carcinogenic metals induce aneuploidy. Here we review the mechanisms for how carcinogenic metals may induce aneuploidy and the evidence that carcinogenic metals induce an aneugenic effect which can destabilize the genome leading to genomic instability and cancer. PMID:21118142

Wise, Sandra S.; Wise, John Pierce

2014-01-01

339

Metal-Insulator Transition of CuO in YBCO\\/CuO\\/YBCO Junction  

Microsoft Academic Search

A resistive transition of CuO sandwiched with Y1Ba2Cu3O7-delta films has been studied. It shows a zero resistive transition around the critical temperature of Y1Ba2Cu3O7-delta. The metal-insulator (or high resistivity) transition has been observed for thicker samples depending on the measuring current.

T. Ogushi; S. Koba; M. Hirose; S. Higo; I. Kawano; A. Nakao

1995-01-01

340

Explicitly correlated composite thermochemistry of transition metal species  

NASA Astrophysics Data System (ADS)

Atomization energies were calculated using explicitly correlated coupled cluster methods with correlation consistent basis sets for a series of 19 small molecules containing 3d transition metal atoms. The atomization energies were calculated using a modified Feller-Peterson-Dixon approach in which CCSD(T) complete basis set (CBS) limits were obtained using extrapolations of aVTZ/aVQZ CCSD(T)-F12b correlation energies, and then a series of additive contributions for relativity, core correlation, higher order correlation, and zero-point vibrations were included. The frozen-core CBS limits calculated with F12 methods closely matched the more computational expensive conventional awCVQZ/awCV5Z CBS extrapolations, with a mean unsigned deviation of just 0.1 kcal/mol. In particular, the CCSD(T*)-F12b/aVDZ and aVTZ atomization energies were more accurate on average than the conventional CCSD(T)/aVQZ and aV5Z results, respectively. In several cases the effects of higher order correlation beyond CCSD(T), as judged by CCSDT and CCSDT(Q)? calculations, were greater than 1 kcal/mol, reaching 4.5 kcal/mol for CrO3. For the 16 molecules of this study with experimental uncertainties of ˜3.5 kcal/mol or less, the final composite heats of formation have a mean unsigned deviation (MUD) from experiment of just 1.3 kcal/mol, which is slightly smaller than the average of the experimental uncertainties, 1.8 kcal/mol. The root mean square deviation (RMS) is only slightly larger at 1.7 kcal/mol. Without the contributions due to higher order correlation effects, the MUD and RMS rise to 2.1 and 2.8 kcal/mol, respectively. To facilitate the F12 calculations, new (aug-)cc-pVnZ/MP2Fit (n = Q, 5) and (aug-)cc-pwCVTZ/MP2Fit auxiliary basis sets were also developed for the transition metal atoms.

Bross, David H.; Hill, J. Grant; Werner, H.-J.; Peterson, Kirk A.

2013-09-01

341

Nanocrystalline transition metal ferrites: Synthesis, characterization and surface functionalization  

NASA Astrophysics Data System (ADS)

In this thesis, a new synthetic methodology for the high yield synthesis of spinel-type transition metal ferrite nanoparticles has been developed. This approach is based on the complexation of the first-row transition metal cations with diethylene glycol (DEG) followed by the hydrolysis of the resulting chelate iron alkoxide complexes in the presence of an alkaline hydroxide. Due to the passivation of their surfaces with DEG molecules, the as-prepared nanoparticles are stable against agglomeration and can be easily dispersed in polar protic solvents (water, alcohols, etc.). Alternatively, a postsynthesis passivation with carboxylate ions can render the iron oxide nanocrystals highly dispersible in non-polar solvents. Optimization of the reaction conditions suggested that the size of the nanocrystals could be controlled by changing the complexing strength of the reaction medium. This hypothesis was verified in the case of the Fe3O4 nanoparticles: their sizes vary from 5.7 nm when the reaction is performed in neat diethylene glycol to 16.8 nm in N-methyl diethanolamine (NMDEA), whereas a 1:1 (%wt) mixture of these solvents yields nanocrystals with an average size of and 12.7 nm. A detailed characterization by using a wide variety of techniques, including powder X-Ray diffraction, IR spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and 1H-NMR spectrometry was performed in order to elucidate the composition and the morphology of the variable-sized iron oxide nanoparticles. Both finite size and interparticle interaction effects were identified to influence the magnetic behavior of the oleate-capped nanosized particles. At low temperatures the Fe3O 4 nanocrystals exhibit a ferromagnetic behavior with blocking temperatures which increase with the average particle size, whereas at room temperature, except for the largest nanoparticles, they undergo a superparamagnetic relaxation. We exploited the high surface reactivity of the 10 nm Fe3O 4 nanoparticles to attach 2-3 nm gold grains to their surfaces through a simple, two-step chemically controlled procedure. By chemically bonding bioactive molecules to the attached Au nanoparticles these novel nanoarchitectures open up new opportunities for the implementation of the magnetic nanoparticles as a platform for various applications in the biomedical field.

Caruntu, Daniela

342

Explicitly correlated composite thermochemistry of transition metal species.  

PubMed

Atomization energies were calculated using explicitly correlated coupled cluster methods with correlation consistent basis sets for a series of 19 small molecules containing 3d transition metal atoms. The atomization energies were calculated using a modified Feller-Peterson-Dixon approach in which CCSD(T) complete basis set (CBS) limits were obtained using extrapolations of aVTZ?aVQZ CCSD(T)-F12b correlation energies, and then a series of additive contributions for relativity, core correlation, higher order correlation, and zero-point vibrations were included. The frozen-core CBS limits calculated with F12 methods closely matched the more computational expensive conventional awCVQZ?awCV5Z CBS extrapolations, with a mean unsigned deviation of just 0.1 kcal?mol. In particular, the CCSD(T?)-F12b?aVDZ and aVTZ atomization energies were more accurate on average than the conventional CCSD(T)?aVQZ and aV5Z results, respectively. In several cases the effects of higher order correlation beyond CCSD(T), as judged by CCSDT and CCSDT(Q)? calculations, were greater than 1 kcal?mol, reaching 4.5 kcal?mol for CrO3. For the 16 molecules of this study with experimental uncertainties of ?3.5 kcal?mol or less, the final composite heats of formation have a mean unsigned deviation (MUD) from experiment of just 1.3 kcal?mol, which is slightly smaller than the average of the experimental uncertainties, 1.8?kcal?mol. The root mean square deviation (RMS) is only slightly larger at 1.7 kcal?mol. Without the contributions due to higher order correlation effects, the MUD and RMS rise to 2.1 and 2.8?kcal?mol, respectively. To facilitate the F12 calculations, new (aug-)cc-pVnZ?MP2Fit (n = Q, 5) and (aug-)cc-pwCVTZ?MP2Fit auxiliary basis sets were also developed for the transition metal atoms. PMID:24028112

Bross, David H; Hill, J Grant; Werner, H-J; Peterson, Kirk A

2013-09-01

343

Comparison of two structures for transition-metal-based half Heusler alloys exhibiting fully compensated half metallicity  

Microsoft Academic Search

We search for new fully compensated half metals, in which only one electronic spin channel is conducting and there exists no net magnetic moment. We focus on half Heusler alloys and we examine the physical consequence of different crystal structures found in the literature for these compounds, XMnZ, with a transition metal element, such as Cr, Mn, and Fe for

Michael Shaughnessy; C. Y. Fong; Lin H. Yang; Claudia Felser

2011-01-01

344

Fingerprints of spin-orbital entanglement in transition metal oxides.  

PubMed

The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the RV O(3) perovskites, with R = La,Pr,…,Y b,Lu, where the finite temperature properties of these compounds can be understood only using entangled states: (i) the thermal evolution of the optical spectral weights, (ii) the dependence of the transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the RV O(3) perovskites, and (iii) the dimerization observed in the magnon spectra for the C-type antiferromagnetic phase of Y V O(3). Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduce topological constraints for the hole propagation and will thus radically modify the transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations. PMID:22776856

Ole?, Andrzej M

2012-08-01

345

Fingerprints of spin-orbital entanglement in transition metal oxides  

E-print Network

The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the $R$VO$_3$ perovskites, with $R$=La,Pr,...,Yb,Lu, where such finite temperature properties of these compounds can be understood only using entangled states: ($i$) thermal evolution of the optical spectral weights, ($ii$) the dependence of transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the $R$VO$_3$ perovskites, and ($iii$) dimerization observed in the magnon spectra for the $C$-type antiferromagnetic phase of YVO$_3$. Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduces topological constraints for the hole propagation and will thus radically modify transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations.

Andrzej M. Ole?

2012-07-12

346

Theoretical studies on layered materials and transition metal borides  

SciTech Connect

From a geometrical point of view, a crystal structural may be described in terms of coordination polyhedra. The number of vertices of polyhedra, the coordination number, depends largely on the cation to anion radius ratio. It is obvious that a minimum coordination number is necessary for the formation of a layer structure. With a coordination number 2, only chain or finite molecules are possible. A high coordination number, on the other hand, will lead to a two, three dimensional or framework structure. The author has studied several compounds which belong to the group of structures mentioned above. V[sub 2]O[sub 5], Bi[sub 2]WO[sub 6], LiBiPd[sub 2]O[sub 4], Pd[sub 3]P2S[sub 8] and CuTe, have a two-dimensional layered structures whereas the hexagonal MT[sub 3]B[sub 2] (M = rare earth metal or Ca, Sr, Ba. T = Fe, Ru, Os, Co, Rh, Ir) are three dimensional expansions of polyhedra. Inside of these solid state crystals, electrons and atoms act collectively to create waves described as quasiparticles. They determine in part whether a crystal is an insulator, a semiconductor, a conductor or a superconductor. In analyzing the electronic structure and properties of the target compounds, the focus is on the study of structural distortations. These studies include the inter-layer interaction in a number of systems and the bonding and dynamics of transition metal boride superconductors. The author relies upon the LCAO approach of the chemist and the tight binding approach of solid state physicist.

Seong, S.

1993-01-01

347

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as 'anomalous'. Our present electron-microscopic studies indicate that this anomalous behaviour usually included the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of charge and spin-density waves. When they first appear, the CDWs in the 1T (and 4Hb) polytypes are incommensurate with the lattice. This condition produes a fair amount of gapping in the density of states at the Fermi level. For the simplest case of 1T-TaSe2, the room temperature superlattice is realized when this existing CDW rotates into an orientation for which it then become commensurate. At this first-order transition the Fermi surface energy gapping increases beyond that generated by the incommensurate CDW, as is clearly evident in the electromagnetic properties. For the trigonal prismatically coordinated polytypes, CDW formation is withheld to low temperatures, probably because of the more complex band structures. This CDW state (in the cases measured) would seem at once commensurate, even though the transition is, from a wide variety of experiments, apparently second order. A wide range of doped and intercalated materials have been used to substantiate the presence of CDWs in these compounds, and to clarify the effect that their occurrence has on the physical properties. The observations further demonstrate the distinctiveness of the transition metal dichalcogenide layer compounds, and of the group VA metals in particular.

Wilson, J. A.; di Salvo, F. J.; Mahajan, S.

2001-12-01

348

Oligomeric rare-earth metal cluster complexes with endohedral transition metal atoms  

NASA Astrophysics Data System (ADS)

Comproportionation reactions of rare-earth metal trihalides (RX3) with the respective rare-earth metals (R) and transition metals (T) led to the formation of 22 oligomeric R cluster halides encapsulating T, in 19 cases for the first time. The structures of these compounds were determined by single-crystal X-ray diffraction and are composed of trimers ({T3R11}X15-type, P63/m), tetramers ({T4R16}X28{R4} (P-43m), {T4R16}X20 (P42/nnm), {T4R16}X24(RX3)4 (I41/a) and {T4R16}X23 (C2/m) types of structure) and pentamers ({Ru5La14}2Br39, Cc) of {TRr}n (n=2-5) clusters. These oligomers are further enveloped by inner (Xi) as well as outer (Xa) halido ligands, which possess diverse functionalities and interconnect like oligomers through i-i, i-a and/or a-i bridges. The general features of the crystal structures for these new compounds are discussed and compared to literature entries as well as different structure types with oligomeric T centered R clusters. Dimers and tetramers originating from the aggregation of {TR6} octahedra via common edges are more frequent than trimers and pentamers, in which the {TRr} clusters share common faces.

Steinberg, Simon; Zimmermann, Sina; Brühmann, Matthias; Meyer, Eva; Rustige, Christian; Wolberg, Marike; Daub, Kathrin; Bell, Thomas; Meyer, Gerd

2014-11-01

349

Transition metal dioxygen complexes as intermediates in homogeneous catalytic oxidations  

NASA Astrophysics Data System (ADS)

The formation and main structural properties of superoxo and peroxo complexes are briefly described. These complexes are involved in catalytic oxidations (oxygenations) by dioxygen occurring under mild conditions in the presence of iron, cobalt, manganese, rhodium and other transition metal complexes. Examples of catalytic systems are taken from cytochrome P-450 models with specific reference to the mechanisms involved. Metalloporphyrin-catalysed oxidations of hydrocarbons involve free-radical chain processes in most cases. Added reducing agents modify the mechanism of oxidation by pumping electrons into key intermediates. They help the formation of oxometal species, which may transfer oxygen atoms to substrates, thereby improving the selectivity. Olefin oxidation is a difficult task for the homogeneous catalysts described: only a few working systems are available, based mainly on rhodium phosphine complexes. Peroxometalacycles are presumably formed, whose decomposition involves co-oxidation of a phosphine molecule. The oxidation of hindered phenols catalysed by cobalt (II) Schiff-base complexes points to the involvement of superoxocobalt species abstracting a H-atom from the phenol. Complex reaction patterns emerge from the oxidation of o-disubstituted phenols. The synthetic potential of homogeneous catalytic oxidation is illustrated on examples including various types of organic compounds.

Simándi, Lászlò I.

350

Two-dimensional transition metal dichalcogenide nanosheet-based composites.  

PubMed

Ultrathin two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDs), such as MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, etc., are emerging as a class of key materials in chemistry and electronics due to their intriguing chemical and electronic properties. The ability to prepare these TMD nanosheets in high yield and large scale via various methods has led to increasing studies on their hybridization with other materials to create novel functional composites, aiming to engineer their chemical, physical and electronic properties and thus achieve good performance for some specific applications. In this critical review, we will introduce the recent progress in hybrid nanoarchitectures based on 2D TMD nanosheets. Their synthetic strategies, properties and applications are systematically summarized and discussed, with emphasis on those new appealing structures, properties and functions. In addition, we will also give some perspectives on the challenges and opportunities in this promising research area. PMID:25292209

Tan, Chaoliang; Zhang, Hua

2014-10-01

351

Transition-metal dihalide MX2 as magnetoelectric multiferroics  

NASA Astrophysics Data System (ADS)

Magnetoelectric properties were investigated for transition-metal dihalide MX2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. We discovered the emergence of ferroelectric polarization (P) in helimagnetic state for several compounds such as CuCl2 with edge-shared S=1/2 chain and MnI2 with stacked triangular lattice. In the latter material, in-plane magnetic field (H) was found to induce the rearrangement of six possible multiferroic domains. With every 60^o-rotation of H around the c-axis, discontinuous 120^o-flop of P-vector is observed as a result of the flop of magnetic modulation vector (q). In-plane H also alters the stable direction of q-vector from original q <110 > to q <110 > above 3 T, which leads to significant change of P-flop patterns under rotating H. At the critical field region (˜ 3T), due to the enhanced q-flexibility, P-vector smoothly rotates clockwisely twice while H rotates counter-clockwisely only once.

Seki, Shinichiro; Kurumaji, Takashi; Ishiwata, Shintaro; Matsui, Hiroyuki; Tokura, Yoshinori; Murakawa, Hiroshi; Tokunaga, Yusuke; Kaneko, Yoshio; Hasegawa, Tatsuo

2011-03-01

352

Destabilization effect of transition metal fluorides on sodium borohydride.  

PubMed

The effect of transition metal fluorides on the decomposition of NaBH4 has been investigated for NaBH4 ball milled with TiF3, MnF3 or FeF3. The compounds were examined by thermal programmed desorption with residual gas analysis, thermo gravimetric analysis and volumetric measurements using a Sieverts-type apparatus. The phase formation process during thermal decomposition was studied by in situ synchrotron radiation powder X-ray diffraction on the as-milled powders. NaBF4 was among the products in all mechano-chemical reactions. (11)B-NMR spectra analysis gave NaBF4?:?NaBH4 ratios of 1?:?150 for Na-Ti, 1?:?40 for Na-Mn, and 1?:?10 for Na-Fe. Pure NaBH4 possessed a hydrogen release onset temperature of 430 °C. The hydrogen release in the NaBH4-MnF3 system began as low as 130 °C. FeF3 decreased the onset temperature to 161 °C and TiF3 to 200 °C. TiF3 reacted completely with NaBH4 below 320 °C. All the examined systems have negligible emissions of diborane species. H-sorption studies performed at selected temperatures above 300 °C exhibited relatively fast desorption kinetics. Partial hydrogen re-absorption was observed for the Na-Mn and Na-Fe samples. PMID:25140831

Kalantzopoulos, Georgios N; Guzik, Matylda N; Deledda, Stefano; Heyn, Richard H; Muller, Jiri; Hauback, Bjørn C

2014-10-14

353

Metal-insulator transitions in IZO, IGZO, and ITZO films  

NASA Astrophysics Data System (ADS)

In this study, we measured the low-temperature resistivity of amorphous two- and three-dimensional (2D and 3D) indium-zinc oxide, indium-gallium-zinc oxide, and indium-tin-zinc oxide films with a wide range of carrier densities. To determine their critical characteristics at the metal-insulator transition (MIT), we used the Ioffe-Regel criterion. We found that the MIT occurs in a narrow range between k F ? = 0.13 and k F ? = 0.25, where k F and ? are the Fermi wave number and electron mean free path, respectively. For films in the insulating region, we analyzed ? ( T ) using a procedure proposed by Zabrodskii and Zinov'eva. This analysis confirmed the occurrence of Mott and Efros-Shklovskii (ES) variable-range hopping. The materials studied show crossover behavior from exp(TMott/T)1/4 or exp(TMott/T)1/3 for Mott hopping conduction to exp(TES/T)1/2 for ES hopping conduction with decreasing temperature. For both 2D and 3D materials, we found that the relationship between TMott and TES satisfies TES ? TMott2/3.

Makise, Kazumasa; Hidaka, Kazuya; Ezaki, Syohei; Asano, Takayuki; Shinozaki, Bunju; Tomai, Shigekazu; Yano, Koki; Nakamura, Hiroaki

2014-10-01

354

Proton extraction by laser ablation of transition metals  

NASA Astrophysics Data System (ADS)

A study on the proton beams extraction from a plasma generated by pulsed laser ablation by targets containing transition metals is presented. The targets used were pure disks of titanium and tantalum and disks of TiH2, obtained by compression of TiH2 powder. The plasma was produced by means of a nanosecond excimer KrF laser operating at low irradiance (109-1010 W/cm2). The proton and ions emission was analyzed by the time-of-flight technique using a Faraday cup as ion collector. Studies on the produced protons and ions at different laser irradiances from 2 to 15 GW/cm2 were performed. The characterization showed that it is possible to obtain good proton fluxes from these targets, up to 1011 proton/pulse. The results obtained are very interesting if compared with those available in literature where proton fluxes per pulse ranging from 108 to 109 by hydride targets were obtained, at the same laser irradiances.

Velardi, L.; Delle Side, D.; Krása, J.; Nassisi, V.

2014-07-01

355

Metal-insulator transitions in IZO, IGZO, and ITZO films  

SciTech Connect

In this study, we measured the low-temperature resistivity of amorphous two- and three-dimensional (2D and 3D) indium-zinc oxide, indium-gallium-zinc oxide, and indium-tin-zinc oxide films with a wide range of carrier densities. To determine their critical characteristics at the metal-insulator transition (MIT), we used the Ioffe–Regel criterion. We found that the MIT occurs in a narrow range between k{sub F}??=?0.13 and k{sub F}??=?0.25, where k{sub F} and ? are the Fermi wave number and electron mean free path, respectively. For films in the insulating region, we analyzed ?(T) using a procedure proposed by Zabrodskii and Zinov'eva. This analysis confirmed the occurrence of Mott and Efros–Shklovskii (ES) variable-range hopping. The materials studied show crossover behavior from exp(T{sub Mott}/T){sup 1/4} or exp(T{sub Mott}/T){sup 1/3} for Mott hopping conduction to exp(T{sub ES}/T){sup 1/2} for ES hopping conduction with decreasing temperature. For both 2D and 3D materials, we found that the relationship between T{sub Mott} and T{sub ES} satisfies T{sub ES}???T{sub Mott}{sup 2/3}.

Makise, Kazumasa, E-mail: makise@nict.go.jp [National Institute of Information and Communications Technology, Kobe 651-2492 (Japan); Hidaka, Kazuya; Ezaki, Syohei; Asano, Takayuki; Shinozaki, Bunju [Department of Physics, Kyushu University, Fukuoka 810-8560 (Japan); Tomai, Shigekazu; Yano, Koki; Nakamura, Hiroaki [Central Research Laboratories, Idemitsu Kosan Co. Ltd, Chiba 299-0293 (Japan)

2014-10-21

356

Exchange Interaction of Transition Metal Dopants in Diamond  

NASA Astrophysics Data System (ADS)

Advances in single-ion implantation and spectroscopy have permitted direct observation of the exchange interaction between two dopant spins in a semiconductor[1], which is accurately described by tight-binding models of the semiconducting host[1,2]. These advances suggest controllable fabrication and utilization of few-dopant structures to explore fundamental properties and for applications[3]. Transition metal substitutional dopants in tetrahedrally-bonded semiconductors are good candidates for controllable spin manipulation and spin-spin interaction because they offer both highly-localized and much more extended spin-polarized states. For example, both the Ni and Cr dopant have spin-1 ground states in diamond, but with differing spatial extent[4]. We calculate the exchange interaction between pairs of Ni and Cr dopants in diamond using the technique of Ref. 2, but with an spds* tight-binding model. We find strong exchange interactions between pairs of Ni, and pairs of Cr, which are influenced by the differing symmetry of the dopants' ground state. [1] D. Kitchen et al., Nature 442, 436 (2006). [2] J.-M. Tang & M.E. Flatté, Phys. Rev. Lett. 92, 047201 (2004). [3] P. Koenraad & M.E. Flatté, Nat. Mat. 10, 91 (2011). [4] T. Chanier, et. al., Phys. Rev. B 86, 085203 (2012).

Kortan, Victoria; Sahin, Cuneyt; Flatté, Michael E.

2013-03-01

357

APCVD Transition Metal Oxides – Functional Layers in "Smart windows"  

NASA Astrophysics Data System (ADS)

Transition metal oxides (TMO) exhibit electrochromic effect. Under a small voltage they change their optical transmittance from transparent to collored (absorbing) state. The individual material can manifest its electrochromic properties only when it is part of electrochromic (EC) multilayer system. Smart window is controlling the energy of solar flux entering the building or car and makes the interiors comfortable and energy utilization more effective. Recently the efforts of material researchers in this field are directed to price decreasing. APCVD technology is considered as promissing as this process permits flowthrough large-scale production process. The paper presents results on device optimization based on WO3-MoO3 working electrode. Extensive research reveals that WO3-MoO3 structure combines positive features of single oxides: excellent electrochromic performance of WO3 and better kinetic properties of MoO3 deposition. The achieved color efficiency of APCVD WO3-MoO3 films is 200cm2/C and optical modulation of 65-70% are practically favorable electrochromic characteristics. To respond to low cost requirement, the expensive hexacarbonyl can be replaced with acetylacetonate. We have started with this precursor to fabricate mixed WxV1-xO3 films. The films possess excellent surface coverage and high growth-rate. CVD deposition of VO2, a promissing thermochromic thin film material is also presented.

Gesheva, K. A.; Ivanova, T. M.; Bodurov, G. K.

2014-11-01

358

Valency configuration of transition metal impurities in ZnO  

SciTech Connect

We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM=Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn{sub 1-x}TM{sub x}O, the localized TM{sup 2+} configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy {epsilon}F close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with {epsilon}F close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.

Petit, Leon [ORNL; Schulthess, Thomas C [ORNL; Svane, Axel [University of Aarhus, Denmark; Temmerman, Walter M [Daresbury Laboratory, UK; Szotek, Zdzislawa [Daresbury Laboratory, UK; Janotti, Anderson [University of California, Santa Barbara

2006-01-01

359

Homology of interatomic forces and Debye temperatures in transition metals  

NASA Astrophysics Data System (ADS)

Using experimental data on the entropy S at intermediate temperatures, corrected for an electronic term to get the vibrational entropy, we obtain well-defined and accurate ``entropy Debye temperatures'' FTHETA=FTHETAS. From FTHETAS we define a quantity with the dimension of a force constant, kS==M(kBFTHETA/?)2, where M is the atomic mass. Similarly, data on the low-temperature vibrational heat capacity and the elastic coefficients yield Debye temperatures FTHETAC and FTHETAelas and corresponding force constants kC and kelas. The ratios of kS for the 4d-5d pairs Zr,Hf, Nb,Ta, Mo,W, Tc,Re, Ru,Os, Rh,Ir, and Pd,Pt are remarkably constant, (kS)4d/(kS)5d =0.76+/-0.01, in spite of varying crystal structures. Further, (kC)4d/(kC)5d =0.70+/-0.07 and (kelas)4d/(kelas)5d =0.69+/-0.08. The corresponding force-constant ratios k3d/k4d vary more. This correlates with an approximately constant atomic-volume ratio ?4d/?5d=0.99+/-0.02, while ?3d/?4d=0.81+/-0.05 shows irregularities, mainly of magnetic origin. As a basis for our analysis, we review low-temperature Debye temperatures FTHETAelas and FTHETAC and the bulk modulus B of the transition metals.

Guillermet, A. Fernández; Grimvall, G.

1989-07-01

360

A computer and analytic study of the metallic liquid-glass transition  

NASA Astrophysics Data System (ADS)

The present work examines two aspects of metallic liquid-glass transitions. First, a computer experiment, consisting of a temperature-quench Monte Carlo calculation for the liquid-glass transition of Ca0.7Mg0.3, is carried out. A comparison with such calculations for monatomic metals indicates that there is a significant difference in the Wendt and Abraham criterion for monatomic metals and metallic alloys. In addition, a comparison with laboratory experiments indicates that the discrepancies between the Wendt-Abraham parameters obtained in these laboratory experiments and in the computer experiments can be ascribed to different cooling rates. Second, an analytic study of the pair distribution function near the transition temperature is presented and reproduces the linear temperature dependence of the Wendt-Abraham parameter. This gives a physical picture for this parameter and indicates that the liquid-glass transition is more likely to be a first-order transition than a second-order one.

Li, D. H.; Moore, R. A.; Wang, S.

1988-02-01

361

Recent advances in transition metal-catalyzed Csp2-monofluoro-, difluoro-, perfluoromethylation and trifluoromethylthiolation  

PubMed Central

Summary In the last few years, transition metal-mediated reactions have joined the toolbox of chemists working in the field of fluorination for Life-Science oriented research. The successful execution of transition metal-catalyzed carbon–fluorine bond formation has become a landmark achievement in fluorine chemistry. This rapidly growing research field has been the subject of some excellent reviews. Our approach focuses exclusively on transition metal-catalyzed reactions that allow the introduction of –CFH2, –CF2H, –CnF2 n +1 and –SCF3 groups onto sp² carbon atoms. Transformations are discussed according to the reaction-type and the metal employed. The review will not extend to conventional non-transition metal methods to these fluorinated groups. PMID:24367416

Landelle, Grégory; Panossian, Armen; Pazenok, Sergiy; Vors, Jean-Pierre

2013-01-01

362

Theory of elastic phase transitions in metals at high pressures. Application to vanadium  

SciTech Connect

Structural transformations in elementary metals under high pressures are considered using the Landau theory of phase transitions, in which the finite strain tensor components play the role of the order parameter. As an example, the phase transition in vanadium observed at a pressure of 69 GPa is analyzed. It is shown that it is a first-order elastic phase transition, which is close to a second-order transition.

Krasil'nikov, O. M., E-mail: omkras@mail.ru; Vekilov, Yu. Kh.; Isaev, E. I.; Bondarenko, N. G. [National Research Technological University MISiS (Russian Federation)

2011-02-15

363

Molecules containing rare-earth atoms solely bonded by transition metals.  

PubMed

Although metal-metal bonding is important in the chemistry of both solid-state intermetallic compounds and molecular species, the study of this bonding is limited by the compounds available and it is rarely possible to identify connections between these two areas. In this study, molecular intermetalloids [Ln(ReCp(2))(3)] (Ln = Sm, Lu and La) have been synthesized that contain lanthanoid metals bound only to transition metals. Although they are highly reactive species, such lanthanoid-core transition-metal-shell compounds can be stable in solution. They mimic the bonding situation of intermetallic compounds, as revealed by a direct comparison of molecular and solid state lanthanoid-transition metal bonding. PMID:20729893

Butovskii, Mikhail V; Döring, Christian; Bezugly, Viktor; Wagner, Frank R; Grin, Yuri; Kempe, Rhett

2010-09-01

364

Molecules containing rare-earth atoms solely bonded by transition metals  

NASA Astrophysics Data System (ADS)

Although metal-metal bonding is important in the chemistry of both solid-state intermetallic compounds and molecular species, the study of this bonding is limited by the compounds available and it is rarely possible to identify connections between these two areas. In this study, molecular intermetalloids [Ln(ReCp2)3] (Ln = Sm, Lu and La) have been synthesized that contain lanthanoid metals bound only to transition metals. Although they are highly reactive species, such lanthanoid-core transition-metal-shell compounds can be stable in solution. They mimic the bonding situation of intermetallic compounds, as revealed by a direct comparison of molecular and solid state lanthanoid-transition metal bonding.

Butovskii, Mikhail V.; Döring, Christian; Bezugly, Viktor; Wagner, Frank R.; Grin, Yuri; Kempe, Rhett

2010-09-01

365

Adjustable metal-semiconductor transition of FeS thin films by thermal annealing  

SciTech Connect

FeS polycrystalline thin films were prepared on float glass at 500 deg. C by radio-frequency reactive sputtering. The influence of vacuum annealing on the metal-semiconductor transition of FeS films was investigated. It has been found that with the increase of the annealing temperature from 360 to 600 deg. C, the metal-semiconductor transition temperature of FeS films first decreases and then increases, associated with first a reduction and then an enhancement of hysteresis width. The thermal stress is considered to give rise to the abnormal change of the metal-semiconductor transition of the FeS film during annealing.

Fu Ganhua; Polity, Angelika; Volbers, Niklas; Meyer, Bruno K.; Mogwitz, Boris; Janek, Juergen [I. Physikalisches Institut, Justus-Liebig-Universitaet Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen (Germany); Physikalisch-Chemisches Institut, Justus-Liebig-Universitaet Giessen, Heinrich-Buff-Ring 58, D-35392 Giessen (Germany)

2006-12-25

366

Microalloying of transition metal silicides by mechanical activation and field-activated reaction  

DOEpatents

Alloys of transition metal suicides that contain one or more alloying elements are fabricated by a two-stage process involving mechanical activation as the first stage and densification and field-activated reaction as the second stage. Mechanical activation, preferably performed by high-energy planetary milling, results in the incorporation of atoms of the alloying element(s) into the crystal lattice of the transition metal, while the densification and field-activated reaction, preferably performed by spark plasma sintering, result in the formation of the alloyed transition metal silicide. Among the many advantages of the process are its ability to accommodate materials that are incompatible in other alloying methods.

Munir, Zuhair A. (Davis, CA); Woolman, Joseph N. (Davis, CA); Petrovic, John J. (Los Alamos, NM)

2003-09-02

367

Transition metal catalyzed carbon-silicon bond forming reactions using chlorosilanes promoted by Grignard reagents.  

PubMed

New catalytic C--Si bond-forming reactions using chlorosilanes are described. These reactions proceed efficiently under mild conditions by the combined use of Grignard reagents and transition metal catalysts, such as Ti, Zr, Ni, and Pd. It is proposed that ate complex intermediates formed by the reaction of transition metals with Grignard reagents play important roles as the active catalytic species. The present study demonstrates the practical use of chlorosilanes in transition metal catalyzed silylation reactions providing convenient methods for allyl- or vinylsilane synthesis. The reaction pathways of these transformations as well as the scope and limitations are discussed. PMID:17318829

Terao, Jun; Kambe, Nobuaki

2007-01-01

368

Incomes and hardship in early transitions to retirement.  

PubMed

ABSTRACT Canada and other countries are changing the age for public pension eligibility. A policy concern is the welfare of those individuals exiting the labour force before the age of pension eligibility. This study, through the use of the Survey of Labour and Income Dynamics data, addressed early retirements by (a) examining the incomes of those who are not working at near-retirement ages, and (b) examining how these Canadians avoid economic hardship. It found that around three-quarters of those not working have been able to avoid low-income status. Most important for avoiding low income are other family income sources, good health, and employment-related pension income. PMID:25511315

Milligan, Kevin

2015-03-01

369

Automatic quantification of early transition points in biofilm formation  

NASA Astrophysics Data System (ADS)

Biofilms are multicellular, dynamic communities of interacting single-cell organisms, like bacteria. Biofilms are responsible for many infectious diseases as well as for significant damage in industrial settings, yet many aspects of biofilm formation are not well understood. Identifying and quantifying the interactions leading to biofilm formation will not only be important for understanding the basic science of these and other multicellular systems, but it will also be essential for designing targeted strategies to prevent or disrupt biofilms. In particular, it is not clear what physical interactions, and corresponding biological mechanisms, are responsible for the early steps in biofilm formation. Because of this, we are developing high-throughput software techniques to analyze micrograph movies of biofilm formation, from attachment to surfaces through the development of microcolonies. This work will focus on developing software tools to identify and quantify key steps in biofilm formation, first in non-chemotacting systems and later in chemotacting (and autotacting) systems.

Thatcher, Travis; Bienvenu, Samuel; Strain, Shinji; Gordon, Vernita

2010-10-01

370

Quantum phase transition between orbital-selective Mott states in Hund's metals  

NASA Astrophysics Data System (ADS)

We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard U , at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.

Rincón, Julián; Moreo, Adriana; Alvarez, Gonzalo; Dagotto, Elbio

2014-12-01

371

Compositional dependence of elastic moduli for transition-metal oxide spinels  

NASA Astrophysics Data System (ADS)

Spinel phases (AB2O4) are common non-silicate oxides in the Earth's crust and upper mantle. A characteristic of this mineral group is the ability to host a wide range of transition metals. Here we summarize the influence of transition metals (Fe, Zn, and Mn) on the pressure dependence of elastic moduli of related spinels (magnetite, gahnite, and franklinite) using GHz-ultrasonic interferometry. Measurements were carried out up to 10 GPa in diamond-anvil cells using hydrostatic pressure media. Transition metals with unfilled 3d orbitals strongly influence the elastic properties of spinels. Franklinite (Zn,Mn)Fe2O4 and magnetite Fe3O4 with transition metals on both A and B cation sites exhibit pressure-induced mode softening of C44, whereas C44 of gahnite(ZnAl2O4) and spinel (MgAl2O4) exhibit positive pressure derivatives of the shear moduli. Spinels with two transition elements tend to undergo phase changes at a lower pressure than those with none or only one transition metal. Along the Mn-Zn solid solution, the variation of moduli with composition is non-linear, and a mid-range franklinite composition studied here shows a minimum in C44 compared with either end-member: MnFe2O 4 or ZnFe2O4. In general, the linear variation of sound velocity with density (Birch's Law) is followed by spinels, however spinels containing only one or no transition metals follow a distinct slope from those containing transition metals on both A and B sites. The Cauchy relation, 0.5(C12 - C44) = P is fulfilled by spinels with only one or no transition metals, suggesting that that Coulomb interactions dominate. Spinels with two transition metals fail to meet the Cauchy relation, indicating strong directional dependence and covalent character of bonding. The bonding character of transition metals is crucial to understanding the elastic behavior of natural and synthetic spinel solid solutions containing transition metals.

Reichmann, H. J.; Jacobsen, S. D.; Boffa Ballaran, T.

2012-12-01

372

Deconfinement transition dynamics and early thermalization in QGP  

E-print Network

We perform SU(3) Lattice Gauge Theory simulations of the deconfinement transition attempting to mimic conditions encountered in heavy ion collisions. Specifically, we perform a sudden temperature quench across the deconfinement temperature, and follow the response of the system in successive simulation sweeps under spatial lattice expansion and temperature fall-off. In measurements of the Polyakov loop and structure functions a robust strong signal of global instability response is observed through the exponential growth of low momentum modes. Development of these long range modes isotropizes the system which reaches thermalization shortly afterwards, and enters a stage of quasi-equilibrium expansion and cooling till its return to the confinement phase. The time scale characterizing full growth of the long range modes is largely unaffected by the conditions of spatial expansion and temperature variation in the system, and is much shorter than the scale set by the interval to return to the confinement phase. The wide separation of these two scales is such that it naturally results in isotropization times well inside 1 fm/c.

E. T. Tomboulis; A. Velytsky

2005-07-15

373

Electronic and geometric structure of transition-metal nanoclusters  

SciTech Connect

A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N = 55, 135, and 140 atoms; we also used the DMOL code to study 13-atom Pd and Cu clusters, with and without hydrogen. The N = 55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) sub-shell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions ({approx} 1 %) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters ({approx} 2-3% in average nearest neighbor spacing), while the icos surface displays small compressions ({approx} 1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bulk results. Calculations with N = 134 suggest slow changes in relative energy with N. Noting that the 135-atom fcc has a much more open surface than the icos, we also compare N = 140 icos and fcc, the latter forming an octahedron with close packed facets. These icos and fcc clusters have identical average coordinations and the octahedron is found to be preferred for Ru and Pd but not for Ag. Finally, we compare Harris functional and LDA energy differences on the N = 140 clusters, and find fair agreement only for Ag.

Jennison, D.R.; Schultz, P.A.; Sears, M.P.; Klitsner, T.

1996-08-01

374

Properties of molecular beam epitaxy grown Eu{sub x}(transition metal){sub y} films (transition metals: Mn, Cr)  

SciTech Connect

The electronic and crystallographic structures, as well as the magnetic properties, of Eu{sub x}(transition metal){sub y} (transition metals: Mn, Cr) thin films grown by molecular beam epitaxy were studied. Relative changes of the Eu/Mn and Eu/Cr ratios derived from the XPS lines, as well as x-ray reflectivity, indicate mixing of the Eu/Mn and Eu/Cr layers. Valency transitions from Eu{sup 2+} to Eu{sup 3+} were observed in both systems for most studied stoichiometries. A transition to a magnetically ordered phase was observed at 15 K, 40 K, and 62 K for selected films in the Eu-Mn system, and at 50 K for the film with a Eu/Cr ratio of 0.5.

Balin, K. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918 (United States); Nowak, A. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Laboratoire de Physique de l'Etat Condense, University du Maine, Le Mans Cedex, 72085 (France); Gibaud, A. [Laboratoire de Physique de l'Etat Condense, University du Maine, Le Mans Cedex, 72085 (France); Szade, J. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Celinski, Z. [Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918 (United States)

2011-04-01

375

The Use of Soluble Polyolefins as Supports for Transition Metal Catalysts  

E-print Network

lab has had interest in the development of soluble polymer-supports for transition metal catalysts. We have developed several separation methods for these soluble polymer-bound catalysts. These include thermomorphic liquid/liquid and solid...

Hobbs, Christopher Eugene

2012-10-19

376

Probing the electron states and metal-insulator transition mechanisms in molybdenum disulphide vertical heterostructures  

NASA Astrophysics Data System (ADS)

The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron–electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.

Chen, Xiaolong; Wu, Zefei; Xu, Shuigang; Wang, Lin; Huang, Rui; Han, Yu; Ye, Weiguang; Xiong, Wei; Han, Tianyi; Long, Gen; Wang, Yang; He, Yuheng; Cai, Yuan; Sheng, Ping; Wang, Ning

2015-01-01

377

Multi-body forces and the energetics of transition metals, alloys, and semiconductors  

SciTech Connect

Progress over the past year is divided into 3 areas: potential-energy functions for transition-metal aluminides; electronic structure and energetics of complex structures and quasicrystals; and ceramic materials (PdO, PtO).

Carlsson, A.E.

1992-01-01

378

Group theory analysis of phonons in two-dimensional transition metal dichalcogenides  

E-print Network

Transition metal dichalcogenides (TMDCs) have emerged as a new two-dimensional material's field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk ...

Ribeiro-Soares, J.

379

Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure  

SciTech Connect

The ion-exchange selectivity of divalent transition metal ions on cryptomelane-type manganic acid (CMA) with tunnel structure has been studied using the distribution coefficients ([ital K][sub [ital d

Tsuji, M. (Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152 (Japan)); Komarneni, S. (Materials Research Laboratory and Department of Agronomy, The Pennsylvania State University, University Park, Pennsylvania 16802-4801 (United States))

1993-03-01

380

Activation of methane by transition metal-substituted aluminophosphate molecular sieves  

DOEpatents

Aluminophosphate molecular sieves substituted with cobalt, manganese or iron and having the AlPO.sub.4 -34 or AlPO.sub.4 -5, or related AlPO.sub.4 structure activate methane starting at approximately 350.degree. C. Between 400.degree. and 500.degree. C. and at methane pressures .ltoreq.1 atmosphere the rate of methane conversion increases steadily with typical conversion efficiencies at 500.degree. C. approaching 50% and selectivity to the production of C.sub.2+ hydrocarbons approaching 100%. The activation mechanism is based on reduction of the transition metal(III) form of the molecular sieve to the transition metal(II) form with accompanying oxidative dehydrogenation of the methane. Reoxidation of the - transition metal(II) form to the transition metal(III) form can be done either chemically (e.g., using O.sub.2) or electrochemically.

Iton, Lennox E. (Downers Grove, IL); Maroni, Victor A. (Naperville, IL)

1991-01-01

381

First-principles density functional theory study of sulfur oxide chemistry on transition metal surfaces  

E-print Network

In this thesis, the chemistry of sulfur oxides on transition metals is studied extensively via first-principles density functional theory (DFT) computations, focusing on the chemical reactivity and selectivity in sulfur ...

Lin, Xi, 1973-

2003-01-01

382

Transition-metal silicides as materials for magnet-semiconductor heterostructures*  

E-print Network

investigated the stability and magnetic properties of thin films of the Heusler alloy Co2MnSi, as well as of binary late transition metal monosilicides, in contact with the Si surface. For the Heusler alloy Co2Mn

383

Electron Spin Resonance of Tetrahedral Transition Metal Oxyanions (MO4n-) in Solids.  

ERIC Educational Resources Information Center

Outlines general principles in observing sharp electron spin resonance (ESR) lines in the solid state by incorporating the transition metal ion of interest into an isostructural diamagnetic host material in small concentration. Examples of some recent studies are described. (CS)

Greenblatt, M.

1980-01-01

384

Transition metal gettering studies and simulation for the optimization of silicon photovoltaic device processing  

E-print Network

We use what is known about transition metal (TM) defect thermodynamic driving forces and kinetic responses to make predictive simulation of gettering during solar cell fabrication possible. We have developed a simulator ...

Smith, Aimée Louise, 1971-

2002-01-01

385

Foundation Degree to Honours Degree: The Transition Experiences of Students on an Early Years Programme  

ERIC Educational Resources Information Center

This article focuses on an exploratory study, undertaken in 2009-2012, which explored student transitions from a foundation degree (level 5) into the third year of a BA honours degree (level 6). Direct entry students and staff from an early years programme at a post-1992 British university and second-year foundation degree students and staff from…

Morgan, Julia

2015-01-01

386

Engaging Fathers from Disadvantaged Areas in Children's Early Educational Transitions: A UK Perspective  

ERIC Educational Resources Information Center

Findings presented here relate to the evaluation of an innovative one-year pilot project in an area of multiple deprivation in the north of England, the goal of which was to engage fathers and male carers in their children's transition from an early years setting to formal schooling to enhance children's enjoyment, achievement and learning at this…

Potter, Carol; Walker, Gary; Keen, Bev

2012-01-01

387

Mothers' Knowledge of Early Adolescents' Activities following the Middle School Transition and Pubertal Maturation  

ERIC Educational Resources Information Center

This study tested a sequential mediation model to determine whether experiences, social cognitions, or parent-adolescent interactional processes account for lower levels of mothers' knowledge of adolescents' whereabouts and activities following early adolescents' transition into middle school (MS) and pubertal development. Cross-sectional data…

Laird, Robert D.; Marrero, Matthew D.

2011-01-01

388

A Stage-Sequential Model of Reading Transitions: Evidence from the Early Childhood Longitudinal Study  

ERIC Educational Resources Information Center

This study uses latent transition analysis to examine reading development across the kindergarten and 1st-grade year. Data include poverty status and dichotomous measures of reading at 4 time points for a large sample of children within the Early Childhood Longitudinal Study. In each of 4 waves of the study, 5 latent classes were represented in…

Kaplan, David; Walpole, Sharon

2005-01-01

389

Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation  

DOEpatents

The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the producer gas from coal gasification processes. 3 figs.

Lilga, M.A.; Hallen, R.T.

1990-08-28

390

Preparation of transition metal nanoparticles and surfaces modified with (CO) polymers synthesized by RAFT  

DOEpatents

A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surface modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a collidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as fuctionalization with a variety of different chemical groups, expanding their utility and application.

McCormick, III, Charles L. (Hattiesburg, MS); Lowe, Andrew B. (Hattiesburg, MS); Sumerlin, Brent S. (Pittsburgh, PA)

2006-10-25

391

Structural and magnetic properties of a variety of transition metal incorporated DNA double helices.  

PubMed

By using density functional theory calculations, the structural, energetic, magnetic, and optical properties for a variety of transition metal (M = Mn, Fe, Co, Ni and Cu) ions incorporated modified-DNA (M-DNA) double helices has been investigated. The DNA is modified with either hydroxypyridone (H) or bis(salicylaldehyde)ethylenediamine (S-en) metalated bases. We find the formation of extended M-O network leading to the ferromagnetic interactions for the case of H-DNA for all the metal ions. More ordered stacking arrangement was found for S-en-DNA. We calculate the exchange coupling constant (J) considering Heisenberg Hamiltonian for quantitative description of magnetic interactions. The ferromagnetic and antiferromagnetic interactions are obtained by varying different transition metal ions. The extent of the magnetic interaction depends on the number of transition metal ions. Optical profiles show peaks below 2?eV, a clear signature of spin-spin coupling. PMID:24382746

Samanta, Pralok K; Pati, Swapan K

2014-02-01

392

Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation  

DOEpatents

The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately .pi.-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancilliary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H.sub.2 from mixed gas streams such as the produce gas from coal gasification processes.

Lilga, Michael A. (Richland, WA); Hallen, Richard T. (Richland, WA)

1990-01-01

393

Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation  

DOEpatents

The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the product gas from coal gasification processes. 3 figures.

Lilga, M.A.; Hallen, R.T.

1991-10-15

394

Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation  

DOEpatents

The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately .pi.-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancilliary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H.sub.2 from mixed gas streams such as the product gas from coal gasification processes.

Lilga, Michael A. (Richland, WA); Hallen, Richard T. (Richland, WA)

1991-01-01

395

Preparation of transition metal nanoparticles and surfaces modified with (co)polymers synthesized by RAFT  

DOEpatents

A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surfaces modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a colloidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as functionalization with a variety of different chemical groups, expanding their utility and application.

McCormick, III, Charles L. (Hattiesburg, MS); Lowe, Andrew B. (Hattiesburg, MS); Sumerlin, Brent S. (Pittsburgh, PA)

2011-12-27

396

Highly Electrical Conductivity of Hybrid Langmuir-Blodgett Films of Transition Metal Dichalcogenide and Amphiphilic Compounds  

Microsoft Academic Search

Hybrid alternate layered films of transition metal dichalcogenides and amphiphilic compounds were prepared by Langmuir-Blodgett (LB) technique. The conductivity at room temperature depended on the transition metal dichalcogenide species, showing the highest value for the hybrid LB films of MoS2 system. This methodology was successfully applied to the fabrication of the hybrid LB films using various organic amphiphiles such as

Hiroaki Tachibana; Yasushi Yamanaka; Hideki Sakai; Masahiko Abe; Mutsuyoshi Matsumoto

2000-01-01

397

Inelastic Scattering Time and Metal-Insulator Transition in Thick Disordered Bismuth Films  

Microsoft Academic Search

The inelastic scattering time of electrons and the metal-insulator transition are studied in disordered thick bismuth films. In weakly disordered films, the anti-localization effect is observed, and dimensional crossover of the inelastic scattering between two and three is demonstrated by changing the thickness of the films. The critical exponent of the metal-insulator transition in this system which represents a disordered

Fumio Komori; Satoshi Okuma; Shun-ichi Kobayashi

1987-01-01

398

Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition.  

PubMed

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases. PMID:24632721

Mundy, Julia A; Hikita, Yasuyuki; Hidaka, Takeaki; Yajima, Takeaki; Higuchi, Takuya; Hwang, Harold Y; Muller, David A; Kourkoutis, Lena F

2014-01-01

399

Metal-insulator transition in ferromagnetic Mn-doped CuO thin films  

NASA Astrophysics Data System (ADS)

The ac susceptibility, electrical properties, and magnetotransport properties of the as-prepared MnxCu1-xO thin films (x =7%-29%) were studied in the temperature range of 2-300 K. The susceptibility measurement shows that ferromagnetic transition takes place for the samples with x ?14% below 100 K. The transport properties show the transition from variable-range-hopping to metalliclike behavior at the same transition temperature as the paramagnetism to ferromagnetism transition. The metal-insulator transition is a characteristic feature of magnetic ordering in this material.

Zhang, Yaping; Pan, Liqing; Gu, Yousong; Zhao, Fan; Qiu, Hongmei; Yin, Jinhua; Zhu, Hao; Xiao, John Q.

2009-04-01

400

ELECTRONIC PROPERTIES OF TRANSITION METAL DICHALCOGENIDES : CONNECTION BETWEEN STRUCTURAL INSTABILITIES AND SUPERCONDUCTIVITY (+)  

Microsoft Academic Search

We discuss the electronic properties of the transition metal dichalcogenide layer compounds which have been investigated in detail because they undergo phase transitions accom- panied by the occurence of charge density waves (CDW). The CDW state is favoured by the cylin- drical nature of the Fermi surface and also by the possible existence of Van-Hove singularities in the band structure

D. JEROME; C. BERTHIER; P. MOLINIE; J. ROUXEL

401

Landau theory of charge-density waves in transition-metal dichalcogenides  

Microsoft Academic Search

A Landau theory is proposed for charge-density waves (CDW) in transition-metal dichalcogenides with the charge density as an order parameter. The theory predicts the sequence of phases, normal-state-incommensurate-CDW-commensurate-CDW with decreasing temperature, separated by first-order phase transitions. The peaks in charge density lie at \\

W. L. McMillan

1975-01-01

402

Transmission spectra of some transition metal dichalcogenides. I. Group IVA: octahedral coordination  

Microsoft Academic Search

The transmission spectra of the layer type group IVA transition metal dichalcogenides have been measured at liquid helium temperature. A great deal more fine structure has been found in these spectra than in those of earlier work, due to interband transitions which have not been observed previously. In all the spectra there is a broad high absorption region and the

A. R. Beal; J. C. Knights; W. Y. Liang

1972-01-01

403

Semiconductor-to-metallic phase transitions in VO2\\/gold nanocomposite materials  

Microsoft Academic Search

Vanadium dioxide is well known for its thermochromic characteristics arising from a semiconductor to metallic phase transition at 68 degree(s)C. The optical properties of the material on each side of the phase transition are relatively well established and the change in refractive index as a result of heating allows the exploitation of the material in fast optical switching devices. Whilst

Keith L. Lewis; Andrew M. Pitt; Richard M. Bennett

2000-01-01

404

Optical studies of some electrolytically produced intercalation complexes of group VA transition metal dichalcogenides  

Microsoft Academic Search

Intercalation complexes of the metallic transition metal dichalcogenides NbS2, NbSe2 and TaS2 with copper, nickel and pyridine have been prepared by an electrolytic technique. The changes in the optical spectra of the host materials after intercalation have been interpreted in terms of charge transfer and the rigid band model for the host materials.

A. R. Beal; W. Y. Liang; J. B. Pethica

1976-01-01

405

A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal oxides  

Microsoft Academic Search

The advent of new methods to prepare semiconductor and metal nanocrystals, specifically the injection of molecular precursors into hot organic surfactants, has yielded markedly improved samples with good size control, narrow size distribution, and good crystallinity of individual and dispersable nanocrystals. It is of considerable interest to apply these methods to the synthesis of transition metal oxide nanoparticles, which typically

Jörg Rockenberger; Erik C. Scher; A. Paul Alivisatos

1999-01-01

406

Between a metal and an insulator: the critical state of the Anderson transition Gabriel Lemarie,1,  

E-print Network

Between a metal and an insulator: the critical state of the Anderson transition Gabriel Lemari´e,1 experimentally realized with a cold atomic gas, we study the transport properties at the critical point of the metal-insulator Anderson transi- tion. We accurately measure the time-evolution of an initially

Paris-Sud XI, Université de

407

Oxygen Auger spectra of some transition-metal oxides: relaxation energies and d-band screening  

Microsoft Academic Search

A number of transition-metal oxides have been investigated by ESCA and special attention has been paid both to the valence band and to the oxygen Auger spectra. Lang and Williams' Auger parameters (1979) have been calculated and the results show that large screening effects, attributable to the metal d band, take place provided the d band is not separated from

P. Humbert; J. P. Deville

1987-01-01

408

Manipulation of a Schlenk Line: Preparation of Tetrahydrofuran Complexes of Transition-Metal Chlorides  

ERIC Educational Resources Information Center

Before taking an inorganic laboratory course few students have experience handling air-sensitive materials using Schlenk techniques. This exercise introduces them to techniques they will employ in later syntheses. The procedure involves the formation of anhydrous tetrahydrofuran complexes of transition-metal chlorides from metal-chloride hydrates;…

Davis, Craig M.; Curran, Kelly A.

2007-01-01

409

Transition metal ions at the crossroads of mucosal immunity and microbial pathogenesis  

PubMed Central

Transition metal ions are essential micronutrients for all living organisms. In mammals, these ions are often protein-bound and sequestered within cells, limiting their availability to microbes. Moreover, in response to infection, mammalian hosts further reduce the availability of metal nutrients by activating epithelial cells and recruiting neutrophils, both of which release metal-binding proteins with antimicrobial function. Microorganisms, in turn, have evolved sophisticated systems to overcome these limitations and acquire the metal ions essential for their growth. Here we review some of the mechanisms employed by the host and by pathogenic microorganisms to compete for transition metal ions, with a discussion of how evading “nutritional immunity” benefits pathogens. Furthermore, we provide new insights on the mechanisms of host-microbe competition for metal ions in the mucosa, particularly in the inflamed gut. PMID:24478990

Diaz-Ochoa, Vladimir E.; Jellbauer, Stefan; Klaus, Suzi; Raffatellu, Manuela

2013-01-01

410

Transition metal ions at the crossroads of mucosal immunity and microbial pathogenesis.  

PubMed

Transition metal ions are essential micronutrients for all living organisms. In mammals, these ions are often protein-bound and sequestered within cells, limiting their availability to microbes. Moreover, in response to infection, mammalian hosts further reduce the availability of metal nutrients by activating epithelial cells and recruiting neutrophils, both of which release metal-binding proteins with antimicrobial function. Microorganisms, in turn, have evolved sophisticated systems to overcome these limitations and acquire the metal ions essential for their growth. Here we review some of the mechanisms employed by the host and by pathogenic microorganisms to compete for transition metal ions, with a discussion of how evading "nutritional immunity" benefits pathogens. Furthermore, we provide new insights on the mechanisms of host-microbe competition for metal ions in the mucosa, particularly in the inflamed gut. PMID:24478990

Diaz-Ochoa, Vladimir E; Jellbauer, Stefan; Klaus, Suzi; Raffatellu, Manuela

2014-01-01

411

Quantum criticality around metal-insulator transitions of strongly correlated electrons  

Microsoft Academic Search

Quantum criticality of metal-insulator transitions in correlated electron systems is shown to belong to an unconventional universality class with violation of Ginzburg-Landau-Wilson (GLW) scheme formulated for symmetry breaking transitions. This unconventionality arises from an emergent character of the quantum critical point, which appears at the marginal point between the Ising-type symmetry breaking at nonzero temperatures and the topological transition of

Takahiro Misawa; Masatoshi Imada

412

Quantum criticality around metal-insulator transitions of strongly correlated electron systems  

Microsoft Academic Search

Quantum criticality of metal-insulator transitions in correlated electron systems is shown to belong to an unconventional universality class with violation of the Ginzburg-Landau-Wilson (GLW) scheme formulated for symmetry breaking transitions. This unconventionality arises from an emergent character of the quantum critical point, which appears at the marginal point between the Ising-type symmetry breaking at nonzero temperatures and the topological transition

Takahiro Misawa; Masatoshi Imada

2007-01-01

413

Trace Metals in the Early Ocean, Biological Implications, and Evolving Biospheric Redox  

NASA Astrophysics Data System (ADS)

Dissolved molybdenum abounds in modern, oxygen-rich seawater as the highly soluble molybdate ion. But this trace metal, also a nutrient essential to phytoplankton productivity, was not always so readily available. Several periods of the geologic past are noted for being euxinic (i.e., oxygen-poor and sulfide-rich) at regional or perhaps even global scales. During the Proterozoic in particular, numerous euxinic settings may have persisted in the ocean for a billion years or more following the initial rise of oxygen in the atmosphere. Under such conditions, Mo is sequestered efficiently, and corresponding limitation in the oceanic inventory may have impacted nitrogen fixation by prokaryotes, as well as uptake of fixed nitrogen by early eukaryotes. Mo is essential for nitrogen fixation and nitrate assimilation as a cofactor for nitrogenase and assimilatory nitrate reductase enzymes, respectively. Evidence for analogous limitations is emerging for portions of the Paleozoic and Mesozoic, but the potential impacts have not yet been considered. In each case, the critical arguments hang on our ability (1) to estimate oceanic extents of oxygen-poor, sulfide-rich settings for the time slice of interest and (2) to discriminate Mo drawdown in the global ocean from that occurring locally within individual basins. Traditional and emerging approaches for distinguishing local from global ancient anoxia/euxinia will be highlighted with an eye toward estimating the impact on availability of Mo and other bioessential, redox-sensitive transition metals. The context for this exploration will be the long history of evolving redox in the ocean and atmosphere from the Archean forward. The Phanerozoic examples are also associated with major biotic extinction events, and our understanding of the relationships among the primary drivers of extinction, widespread euxinic conditions, and the impact of micronutrient availability during extinction and recovery remains nascent.

Lyons, T. W.; Scott, C.; Gill, B. C.; Anbar, A. D.

2008-12-01

414

Transition-metal-ion-mediated polymerization of dopamine: mussel-inspired approach for the facile synthesis of robust transition-metal nanoparticle-graphene hybrids.  

PubMed

Inspired by the high transition-metal-ion content in mussel glues, and the cross-linking and mechanical reinforcement effects of some transition-metal ions in mussel threads, high concentrations of nickel(II), cobalt(II), and manganese(II) ions have been purposely introduced into the reaction system for dopamine polymerization. Kinetics studies were conducted for the Ni(2+)-dopamine system to investigate the polymerization mechanism. The results show that the Ni(2+) ions could accelerate the assembly of dopamine oligomers in the polymerization process. Spectroscopic and electron microscopic studies reveal that the Ni(2+) ions are chelated with polydopamine (PDA) units, forming homogeneous Ni(2+)-PDA complexes. This facile one-pot approach is utilized to construct transition-metal-ion-PDA complex thin coatings on graphene oxide, which can be carbonized to produce robust hybrid nanosheets with well-dispersed metallic nickel/metallic cobalt/manganese(II) oxide nanoparticles embedded in PDA-derived thin graphitic carbon layers. The nickel-graphene hybrid prepared by using this approach shows good catalytic properties and recyclability for the reduction of p-nitrophenol. PMID:24862644

Yang, Liping; Kong, Junhua; Zhou, Dan; Ang, Jia Ming; Phua, Si Lei; Yee, Wu Aik; Liu, Hai; Huang, Yizhong; Lu, Xuehong

2014-06-16

415

X-ray absorption spectroscopy of layered 4D transition-metal dichalcogenides  

Microsoft Academic Search

The authors have measured the metal LIII absorption spectra of layered 4d transition-metal dichalcogenides with the use of a Yohan-type curved-crystal spectrometer. The spectra showed that the metal d orbitals are strongly admixed with the chalcogen p orbitals to form the covalent bonding. The rigid-band model which has been suggested by Wilson and Yoffe (1969) appears to be valid in

Y. Ohno; K. Hirama; S. Nakai; C. Sugiura; S. Okada

1983-01-01

416

Realization and electrical characterization of ultrathin crystals of layered transition-metal dichalcogenides  

Microsoft Academic Search

Ultrathin crystals of the layered transition-metal dichalcogenide MoS2 (semiconducting) and TaS2 (metallic) were obtained by mechanical peeling or chemical exfoliation techniques and electrically contacted using electron-beam lithography. The MoS2 devices showed high field-effect mobility in the tens of cm2\\/V s and an on\\/off ratio higher than 105. The TaS2 devices remained metallic despite the fabrication process and showed an enhancement

Anthony Ayari; Enrique Cobas; Ololade Ogundadegbe; Michael S. Fuhrer

2007-01-01

417

CVD - technology of transition metal oxides and their impact on solar energy utilization  

Microsoft Academic Search

The paper presents results concerning CVD thin films of transition metals (W and Mo) and their composite structures W02:W and MoOZ:Mo, as well as WO films. The composite structure materials 3 consisting of a suspension of metallic molybdenum or tungsten grains in a host matrix of metal dioxide demonstrate significant solar absorptance coupled with a high infrared reflectance, which is

K. A. GESHEVA; D. S. GOGOVA

1993-01-01

418

Critical Behavior of Metal-Insulator Transition in La1-xSrxVO3  

Microsoft Academic Search

Critical behavior of the metal-insulator transition (MIT) coupled with spin\\/orbital correlations has been investigated for single crystals of La1-xSrxVO3. In the paramagnetic (PM) metal phase \\\\(x>0.260\\\\), the precursor to the MIT manifests itself as an enhancement of carrier effective mass. In the antiferromagnetic (AF) metal phase \\\\(0.178<=x<=0.260\\\\), the carrier density decreases and the correlation of the orbital seems to evolve

S. Miyasaka; T. Okuda; Y. Tokura

2000-01-01

419

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial.  

PubMed

Electron-electron interactions can render an otherwise conducting material insulating, with the insulator-metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator-metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340?K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator-metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites. PMID:22801506

Liu, Mengkun; Hwang, Harold Y; Tao, Hu; Strikwerda, Andrew C; Fan, Kebin; Keiser, George R; Sternbach, Aaron J; West, Kevin G; Kittiwatanakul, Salinporn; Lu, Jiwei; Wolf, Stuart A; Omenetto, Fiorenzo G; Zhang, Xin; Nelson, Keith A; Averitt, Richard D

2012-07-19

420

Electronic and vibrational properties of defective transition metal dichalcogenide Haeckelites: new 2D semi-metallic systems  

NASA Astrophysics Data System (ADS)

The electronic properties of monolayered Haeckelites constructed from transition metal dichalcogenides (TMDs) such as MoS2, WS2, WSe2 and MoSe2, have been studied by first principles calculations. It was found that monolayered Haeckelites exhibit a semi-metalic behavior, whereas the traditional monolayered TMDs show a direct band gap, which is characteristic of semiconducting TMDs. Furthermore, metallic TMD monolayers of NbS2 or NbSe2 transform into direct and indirect gap semiconductors, respectively when arranged into Haeckelite geometries. The vibrational properties and Raman spectra of NbS2 Haeckelites were also calculated.

Terrones, H.; Terrones, M.

2014-06-01

421

Insulator-metal transition of highly compressed carbon disulfide  

SciTech Connect

We present integrated spectral, structural, resistance, and theoretical evidences for simple molecular CS{sub 2} transformations to an insulating black polymer with threefold carbon atoms at 9 GPa, then to a semiconducting polymer above 30 GPa, and finally to a metallic solid above 50 GPa. The metallic phase is a highly disordered three-dimensional network structure with fourfold carbon atoms at the carbon-sulfur distance of {approx}1.70 {angstrom}. Based on first-principles calculations, we present two plausible structures for the metallic phase: {alpha}-chalcopyrite and tridymite, both of which exhibit metallic ground states and disordered diffraction features similar to that measured. We also present the phase and chemical transformation diagram for carbon disulfide, showing a large stability field of the metallic phase to 100 GPa and 800 K.

Dias, Ranga P.; Yoo, Choong-Shik; Kim, Minseob; Tse, John S. (WSU); (Saskatchewan)

2012-04-24

422

Barium distributions in teeth reveal early life dietary transitions in primates  

PubMed Central

Early life dietary transitions reflect fundamental aspects of primate evolution and are important determinants of health in contemporary human populations1,2. Weaning is critical to developmental and reproductive rates; early weaning can have detrimental health effects but enables shorter inter-birth intervals, which influences population growth3. Uncovering early life dietary history in fossils is hampered by the absence of prospectively-validated biomarkers that are not modified during fossilisation4. Here we show that major dietary shifts in early life manifest as compositional variations in dental tissues. Teeth from human children and captive macaques, with prospectively-recorded diet histories, demonstrate that barium (Ba) distributions accurately reflect dietary transitions from the introduction of mother’s milk and through the weaning process. We also document transitions in a Middle Palaeolithic juvenile Neanderthal, which shows a pattern of exclusive breastfeeding for seven months, followed by seven months of supplementation. After this point, Ba levels in enamel returned to baseline prenatal levels, suggesting an abrupt cessation of breastfeeding at 1.2 years of age. Integration of Ba spatial distributions and histological mapping of tooth formation enables novel studies of the evolution of human life history, dietary ontogeny in wild primates, and human health investigations through accurate reconstructions of breastfeeding history. PMID:23698370

Austin, Christine; Smith, Tanya M.; Bradman, Asa; Hinde, Katie; Joannes-Boyau, Renaud; Bishop, David; Hare, Dominic J.; Doble, Philip; Eskenazi, Brenda; Arora, Manish

2013-01-01

423

Theory of quantum metal to superconductor transitions in highly conducting systems  

SciTech Connect

We derive the theory of the quantum (zero temperature) superconductor to metal transition in disordered materials when the resistance of the normal metal near criticality is small compared to the quantum of resistivity. This can occur most readily in situations in which 'Anderson's theorem' does not apply. We explicitly study the transition in superconductor-metal composites, in an swave superconducting film in the presence of a magnetic field, and in a low temperature disordered d-wave superconductor. Near the point of the transition, the distribution of the superconducting order parameter is highly inhomogeneous. To describe this situation we employ a procedure which is similar to that introduced by Mott for description of the temperature dependence of the variable range hopping conduction. As the system approaches the point of the transition from the metal to the superconductor, the conductivity of the system diverges, and the Wiedemann-Franz law is violated. In the case of d-wave (or other exotic) superconductors we predict the existence of (at least) two sequential transitions as a function of increasing disorder: a d-wave to s-wave, and then an s-wave to metal transition.

Spivak, B.

2010-04-06

424

Photoluminescence response of colloidal quantum dots on VO2 film across metal to insulator transition  

PubMed Central

We have proposed a method to probe metal to insulator transition in VO2 measuring photoluminescence response of colloidal quantum dots deposited on the VO2 film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO2 across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data. PACS 71.30. + h; 73.21.La; 78.47.jd PMID:25404877

2014-01-01

425

Gas-phase activation of methane by ligated transition-metal cations  

PubMed Central

Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C–H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or ?-bond metathesis. PMID:18955709

Schröder, Detlef; Schwarz, Helmut

2008-01-01

426

Templated Assembly of Metal-Anion Arrays within Layered Hosts; Synthesis and Characterization of New Transition-Metal Oxyhalide Perovskites  

E-print Network

Templated Assembly of Metal-Anion Arrays within Layered Hosts; Synthesis and Characterization], superconductivity [12], ferroelectricity [13], colossal magnetoresistance [14-15] and magnetic transitions-anion arrays within receptive hosts and the formation of new mixed valence compounds. Additionally, we

Spinu, Leonard

427

Correlation between local vibrations and metal mass in AlB2-type transition-metal diborides.  

PubMed

Lattice vibrations have been investigated in TiB2, ZrB2 and HfB2 by temperature-dependent extended X-ray absorption fine structure (EXAFS) experiments. Data clearly show that the EXAFS oscillations are characterized by an anomalous behavior of the Debye-Waller factor of the transition-metal-boron pair, which is suggested to be associated with a superposition of an optical mode corresponding to phonon vibrations induced by the B sublattice and an acoustic mode corresponding to the transition-metal (TM) sublattice. Data can be interpreted as a decoupling of the metal and boron vibrations observed in these transition-metal diborides (TMB2), a mechanism that may be responsible for the significant reduction of the superconducting transition temperature observed in these systems with respect to the parent MgB2 compound. The vibrational behavior of TM-TM bonds has also been investigated to study the occurrence of anisotropy and anomalies in the lattice vibrational behavior of TM-TM bonds. PMID:19096171

Chu, W S; Zhang, S; Yu, M J; Zheng, L R; Hu, T D; Zhao, H F; Marcelli, A; Bianconi, A; Saini, N L; Liu, W H; Wu, Z Y

2009-01-01

428

PHYSICAL REVIEW B 90, 144102 (2014) First-principles study of bismuth films at transition-metal grain boundaries  

E-print Network

of polycrystals such as grain growth [1,2], liquid metal embrittlement (LME) [3,4], and corrosion [5,6]. HoweverPHYSICAL REVIEW B 90, 144102 (2014) First-principles study of bismuth films at transition-metal energies of Bi films on transition-metal (TM) 3(111) and 5(012) GBs using density functional theory. Our

Widom, Michael

429

Electronic structure of mixed valence transition metal oxides Institute of Physics, Academy of Sciences of the Czech Republic,  

E-print Network

Electronic structure of mixed valence transition metal oxides P. Nov´ak1 1 Institute of Physics: 1 #12;I. INTRODUCTION The character of the charge carriers in oxides of transition metals belongs metals. Traditionally oxides were considered to be ionic compounds, in which the oxygen ions have fully

Tebbens, Jurjen Duintjer

430

Oxidation energies of transition metal oxides within the GGA+U framework Lei Wang, Thomas Maxisch, and Gerbrand Ceder*  

E-print Network

Oxidation energies of transition metal oxides within the GGA+U framework Lei Wang, Thomas Maxisch; published 4 May 2006 The energy of a large number of oxidation reactions of 3d transition metal oxides be corrected by fitting the formation enthalpy of simple nontransition metal oxides. Removal of the O2 binding

Ceder, Gerbrand

431

Graphite fountain: Modeling of growth on transition metals under a thermal Jongpil Ye and Rodney S. Ruoff  

E-print Network

Graphite fountain: Modeling of growth on transition metals under a thermal gradient Jongpil Ye://jap.aip.org/about/rights_and_permissions #12;Graphite fountain: Modeling of growth on transition metals under a thermal gradient Jongpil Ye1,a to synthesize graphite as a continuous growth in solid metal cylinders under a thermal gradient through

432

Early Childhood Teachers' Perceived Competence during Transition from Teacher Education to Work: Results from a Longitudinal Study  

ERIC Educational Resources Information Center

The transition from education to work is a challenge for early childhood teachers. In this study, competence self-ratings of 348 German early childhood teachers were investigated one year before, at the end of and four months after early childhood teacher education at universities and vocational schools. Perceived competence was assessed by means…

Mischo, Christoph

2015-01-01

433

Comparative thermal equations of state of state for transition metal (Fe, Ni, Co) ringwoodites and implications for the Earth's chemical evolution  

NASA Astrophysics Data System (ADS)

The concentration of transition metal elements in the metal, oxide, silicate, and melt phases in the deep Earth has a major influence on physical properties such as density and elasticity, and transport properties such as electrical and thermal conductivity and rheology. Additionally, the mantle concentrations of transition metals are reflective of the conditions of core-mantle segregation in the early Earth. The divalent transition metals Fe, Ni and Co all form stable olivine structured minerals and transform to the spinel-ringwoodite structure at high pressures. Here we present measurements of the thermoelastic properties and phase stabilities of a suite of transition metal ringwoodites at pressures and temperatures corresponding to the Earth's mid-mantle. X-ray diffraction measurements were performed using synchrotron X-ray diffraction in conjunction with in situ laser heating in the diamond anvil cell at GSECARS and HPCAT at the Advanced Photon Source. The relative thermoelastic properties of Fe2SiO4, Ni2SiO4, and Co2SiO4 ringwoodite are examined, and compared with relative thermoelastic parameters of FeO, NiO, and CoO to assess the degree to which the metal cation controls the thermoelastic properties of the silicate and oxide phases. In addition, the differences between free energies for transition metal ringwoodites and their corresponding metals are calculated as a function of pressure and temperature using new and existing thermoelastic data. These values are used to assess their relative tendency to oxidize or reduce at high pressures and temperatures.

Armentrout, M. M.; Kavner, A.

2012-12-01

434

Neutral bimetallic transition metal phenoxyiminato catalysts and related polymerization methods  

DOEpatents

A catalyst composition comprising a neutral bimetallic diphenoxydiiminate complex of group 10 metals or Ni, Pd or Pt is disclosed. The compositions can be used for the preparation of homo- and co-polymers of olefinic monomer compounds.

Marks, Tobin J. (Evanston, IL); Rodriguez, Brandon A. (Evanston, IL); Delferro, Massimiliano (Chicago, IL)

2012-08-07

435

Unravelling the switching mechanisms in electric field induced insulator-metal transitions in VO2 nanobeams  

NASA Astrophysics Data System (ADS)

We studied insulator-metal transitions in VO2 nanobeams for both abrupt and gradual changes in applied electric fields. Based on the observations, the Poole-Frenkel effect explained the abrupt transition, while the gradual case is found to be dominated by the Joule heating phenomenon. We also carried out power model and finite element method based simulations which supported the Joule heating phenomena for gradual transition. An in-principle demonstration of the Poole-Frenkel effect, performed using a square voltage pulse of 1 µs duration, further confirms the proposed insulator-metal transition mechanism with a switching time in the order of 100 ns. Finally, conductivity variations introduced via rapid thermal annealing at various temperatures validate the roles of both Joule heating and Poole-Frenkel mechanisms in the transitions.

Rathi, Servin; Park, Jin-Hyung; Lee, In-yeal; Baik, Jeong Min; Yi, Kyung Soo; Kim, Gil-Ho

2014-07-01

436

Differential tolerance of Sulfolobus strains to transition metals  

USGS Publications Warehouse

Sulfolobus acidocaldarius strains 98-3 and B12, and S. solfataricus ATCC 35091 were evaluated for tolerance to Cd, Co, Cu, Ni, Zn and Mg. The tolerance of strains 98-3 and ATCC 35091 to these metals was Mg > Zn > Cd > Cu ??? Co > Ni. For B12, however, the order of tolerance was Mg > Cd > Zn = Co > Ni > Cu. Tolerance to these metals is also presented as a potentially useful taxonomic indicator.

Miller, K.W.; Sass, Risanico S.; Risatti, J.B.

1992-01-01

437

Ternary rare earth and actinoid transition metal carbides viewed as carbometalates  

NASA Astrophysics Data System (ADS)

Ternary carbides A xT yC z ( A=rare earth metals and actinoids; T=transition metals) with monoatomic species C 4- as structural entities are classified according to the criteria (i) metal to carbon ratio, (ii) coordination number of the transition metal by carbon atoms, and (iii) the dimensionality of the anionic network [ T yC z] n-. Two groups are clearly distinguishable, depending on the metal to carbon ratio. Those where this ratio is equal to or smaller than 2 may be viewed as carbometalates, thus extending the sequence of complex anions from fluoro-, oxo-, and nitridometalates to carbometalates. The second group, metal-rich carbides with metal to carbon ratios equal to or larger than 4 is better viewed as typical intermetallics ("interstitial carbides"). The chemical bonding properties have been investigated by analyzing the Crystal Orbital Hamilton Population (COHP). The chemical bonding situation with respect to individual T-C bonds is similar in both classes. The main difference is the larger number of metal-metal bonds in the crystal structures of the metal-rich carbides.

Dashjav, Enkhtsetseg; Kreiner, Guido; Schnelle, Walter; Wagner, Frank R.; Kniep, Rüdiger; Jeitschko, Wolfgang

2007-02-01

438

Metal-insulator transition in Na{sub x}WO{sub 3}: Photoemission spectromicroscopy study  

SciTech Connect

We have investigated the validity of percolation model, which is quite often invoked to explain the metal-insulator transition in sodium tungsten bronzes, Na{sub x}WO{sub 3} by photoelectron spectromicroscopy. The spatially resolved direct spectromicroscopic probing on both the insulating and metallic phases of high quality single crystals of Na{sub x}WO{sub 3} reveals the absence of any microscopic inhomogeneities embedded in the system within the experimental limit. Neither any metallic domains in the insulating host nor any insulating domains in the metallic host have been found to support the validity of percolation model to explain the metal-insulator transition in Na{sub x}WO{sub 3}.

Paul, Sanhita, E-mail: raj@iiserkol.ac.in; Ghosh, Anirudha, E-mail: raj@iiserkol.ac.in; Raj, Satyabrata, E-mail: raj@iiserkol.ac.in [Department of Physical Sciences, Indian Institute of Science Education and Research - Kolkata, Mohanpur Campus, Nadia -741252, West Bengal (India)

2014-04-24

439

Polymer derived non-oxide ceramics modified with late transition metals.  

PubMed

This tutorial review highlights the methods for the preparation of metal modified precursor derived ceramics (PDCs) and concentrates on the rare non-oxide systems enhanced with late transition metals. In addition to the main synthetic strategies for modified SiC and SiCN ceramics, an overview of the morphologies, structures and compositions of both, ceramic materials and metal (nano) particles, is presented. Potential magnetic and catalytic applications have been discussed for the so manufactured metal containing non-oxide ceramics. PMID:22337594

Zaheer, Muhammad; Schmalz, Thomas; Motz, Günter; Kempe, Rhett

2012-08-01

440

Construction of 2D atomic crystals on transition metal surfaces: graphene, silicene, and hafnene.  

PubMed

The synthesis and structures of graphene on Ru(0001) and Pt(111), silicene on Ag(111) and Ir(111) and the honeycomb hafnium lattice on Ir(111) are reviewed. Epitaxy on a transition metal (TM) substrate is a pro-mising method to produce a variety of two dimensional (2D) atomic crystals which potentially can be used in next generation electronic devices. This method is particularly valuable in the case of producing 2D materials that do not exist in 3D forms, for instance, silicene. Based on the intensive investigations of epitaxial graphene on TM in recent years, it is known that the quality of graphene is affected by many factors, including the interaction between the 2D material overlayer and the substrate, the lattice mismatch, the nucleation density at the early stage of growth. It is found that these factors also apply to many other epitaxial 2D crystals on TM. The knowledge from the reviewed systems will shine light on the design and synthesis of new 2D crystals with novel properties. PMID:24687899

Pan, Yi; Zhang, Lizhi; Huang, Li; Li, Linfei; Meng, Lei; Gao, Min; Huan, Qing; Lin, Xiao; Wang, Yeliang; Du, Shixuan; Freund, Hans-Joachim; Gao, Hong-Jun

2014-06-12

441

[Transition metal mediated transformations of small molecules]. Progress report  

SciTech Connect

Work on organotransition metal chemistry, homogeneous and heterogeneous catalysis is summarized. Several cationic palladium(II) complexes with bulky phosphine or pyridine ligands were discovered that are highly selective catalysts for linear dimerization of vinyl monomers and linear polymerization of p-divinylbenzene, the reactions proceeding through a carbocationic mechanism. Our studies were continued on alternating olefin-carbon monoxide copolymers. The copolymerization reaction and reactivity of copolymers were examined. New catalytic systems for alternating copolymerization of {alpha}-olefins with CO were discovered. In the case of styrene derivatives, tactic copolymers were obtained. Poly(ethylenepyrrolediyl) derivatives were synthesized from alternating ethylene-carbon monoxide copolymer and become electronic conductors when doped with iodine. A catalytic system for direct synthesis of polyureas and polyoxamides from and diamines was also discovered. Pt metal catalyzed the oxidation of ethers, esters, and amines to carboxylic acids and the oxidation of olefins to 1,2-diols. Anaerobic and aerobic decomposition of molybdenum(VI)-oxoalkyl compounds were studied for heterogeneous oxidation of alkanes and olefins on Mo(VI)-oxide surfaces. Synthesis of polymer-trapped metal, metal oxide, and metal sulfide nanoclusters (size <1--10 nm) was studied.

Sen, A.

1992-10-01

442

The ReSTAGE Collaboration: Defining Optimal Bleeding Criteria for Onset of Early Menopausal Transition  

PubMed Central

Study objective Criteria for staging the menopausal transition are not established. This paper evaluates five bleeding criteria for defining early transition and provides empirically-based guidance regarding optimal criteria. Design/Setting Prospective menstrual calendar data from four population-based cohorts: TREMIN, Melbourne Women’s Midlife Health Project(MWMHP), Seattle Midlife Women’s Health Study(SMWHS), and Study of Women’s Health Across the Nation(SWAN) with annual serum follicle stimulating hormone (FSH) from MWMHP and SWAN. Participants 735 TREMIN, 279 SMWHS, 216 MWMHP, and 2270 SWAN women aged 35-57 at baseline who maintained menstrual calendars. Main outcome measure(s) Age at and time to menopause for: standard deviation >6 and >8 days, persistent difference in consecutive segments >6 days, irregularity, and >=45 day segment. Serum follicle stimulating hormone concentration. Results Most women experienced each of the bleeding criteria. Except for persistent >6 day difference which occurs earlier, the criteria occur at a similar age and at approximately the same age as late transition in a large proportion of women. FSH was associated with all proposed markers. Conclusions The early transition may be best described by ovarian activity consistent with the persistent >6 day difference, but further study is needed, as other proposed criterion are consistent with later menstrual changes. PMID:17681300

Harlow, Siobán D.; Mitchell, Ellen S.; Crawford, Sybil; Nan, Bin; Little, Roderick; Taffe, John

2008-01-01

443

Ab-initio study of transition metal hydrides  

SciTech Connect

We have performed ab initio self consistent calculations based on Full potential linearized augmented plane wave (FP-LAPW) method to investigate the optical and thermal properties of yttrium hydrides. From the band structure and density of states, the optical absorption spectra and specific heats have been calculated. The band structure of Yttrium metal changes dramatically due to hybridization of Y sp orbitals with H s orbitals and there is a net charge transfer from metal to hydrogen site. The electrical resistivity and specific heats of yttrium hydrides are lowered but the thermal conductivity is slightly enhanced due to increase in scattering from hydrogen sites.

Sharma, Ramesh [Dept. of Physics, Feroze Gandhi Insititute of Engineering and Technology, Raebareli-229001 (India); Shukla, Seema, E-mail: sharma.yamini62@gmail.com; Dwivedi, Shalini, E-mail: sharma.yamini62@gmail.com; Sharma, Yamini, E-mail: sharma.yamini62@gmail.com [Theoretical Condensed Matter Physics Laboratory, Dept. of Physics Feroze Gandhi College, Raebareli-229001 (India)

2014-04-24

444

Theory of a Continuous Stripe Melting Transition in a Two-Dimensional Metal: A Possible Application to Cuprate Superconductors  

E-print Network

We construct a theory of continuous stripe melting quantum phase transitions in two-dimensional metals and the associated Fermi surface reconstruction. Such phase transitions are strongly coupled but yet theoretically ...

Mross, David Fabian

445

Transport mechanisms for iron and other transition metals in rat and rabbit erythroid cells.  

PubMed

1. Earlier studies have shown that Fe2+ transport into erythroid cells is inhibited by several transition metals (Mn2+, Zn2+, Co2+, Ni2+) and that Fe2+ transport can occur by two saturable mechanisms, one of high affinity and the other of low affinity. Also, the transport of Zn2+ and Cd2+ into erythroid cells is stimulated by NaHCO3 and NaSCN. The aim of the present investigation was to determine whether all of these transition metals can be transported by the processes described for Fe2+, Zn2+ and Cd2+ and to determine the properties of the transport processes. 2. Rabbit reticulocytes and mature erythrocytes and reticulocytes from homozygous and heterozygous Belgrade rats were incubated with radiolabelled samples of the metals under conditions known to be optimal for high- and low-affinity Fe2+ transport and for the processes mediated by NaHCO3 and NaSCN. 3. All of the metals were transported by the high- and low-affinity Fe2+ transport processes and could compete with each other for transport. The Km and Vmax values and the effects of incubation temperature and metabolic inhibitors were similar for all the metals. NaHCO3 and NaSCN increased the uptake of Zn2+ and Cd2+ but not the other metals. 4. The uptake of all of the metals by the high-affinity process was much lower in reticulocytes from homozygous Belgrade rats than in those from heterozygous animals, but there was no difference with respect to low-affinity transport. 5. It is concluded that the high- and low-affinity 'iron' transport mechanisms can also transport several other transition metals and should therefore be considered as general transition metal carriers. PMID:9518737

Savigni, D L; Morgan, E H

1998-05-01

446

Generalized self-assembly of scalable two-dimensional transition metal oxide nanosheets  

NASA Astrophysics Data System (ADS)

Two-dimensional (2D) transition metal oxide systems present exotic electronic properties and high specific surface areas, and also demonstrate promising applications ranging from electronics to energy storage. Yet, in contrast to other types of nanostructures, the question as to whether we could assemble 2D nanomaterials with an atomic thickness from molecules in a general way, which may give them some interesting properties such as those of graphene, still remains unresolved. Herein, we report a generalized and fundamental approach to molecular self-assembly synthesis of ultrathin 2D nanosheets of transition metal oxides by rationally employing lamellar reverse micelles. It is worth emphasizing that the synthesized crystallized ultrathin transition metal oxide nanosheets possess confined thickness, high specific surface area and chemically reactive facets, so that they could have promising applications in nanostructured electronics, photonics, sensors, and energy conversion and storage devices.

Sun, Ziqi; Liao, Ting; Dou, Yuhai; Hwang, Soo Min; Park, Min-Sik; Jiang, Lei; Kim, Jung Ho; Dou, Shi Xue

2014-05-01

447

Advances in transition metal-catalyzed asymmetric hydrogenation of heteroaromatic compounds.  

PubMed

Transition metal-catalyzed asymmetric hydrogenation of heteroaromatic compounds is undoubtedly a straightforward and environmentally friendly method for the synthesis of a wide range of optically active heterocyclic compounds, which are widespread and ubiquitous in naturally occurring and artificial bioactive molecules. Over the past decade, a number of transition metal (Ir, Rh, Ru, and Pd) catalysts bearing chiral phosphorus ligands, amine-tosylamine ligands, and N-heterocyclic carbene ligands have been developed for such challenging transformation. This review will describe the significant contributions concerning the transition metal-catalyzed asymmetric hydrogenation of N-, O-, and S-containing heteroaromatic compounds, with emphasis on the evolution of different chiral ligands, related catalyst immobilization, and mechanism investigations. PMID:24173671

He, Yan-Mei; Song, Feng-Tao; Fan, Qing-Hua

2014-01-01

448

Frustrated Lewis pairs beyond the main group: transition metal-containing systems.  

PubMed

The concept of the "frustrated" Lewis pair (FLP) offers a valuable paradigm for transition metal complexes. This chapter describes recent developments in FLP chemistry where one or both of the Lewis acidic or Lewis basic components are based on a transition metal. At the forefront of these developments have been zirconocene-phosphinoaryloxide complexes; the activation of small molecules, including H2, CO2, ethers, olefins and alkyl chlorides or fluorides, with such zirconocene (or group 4 metallocene in general) phosphine pairs is described. Nascent catalytic applications for such complexes in dehydrogenation reactions and future possibilities for catalytic reactivity are also highlighted. A wider discussion of how the FLP concept may rationalise previous examples of cooperative reactivity in transition metal complexes, such as the heterolytic cleavage of hydrogen, suggests a strong and useful analogy. PMID:23468285

Wass, Duncan F; Chapman, Andy M

2013-01-01

449

Prebiotic coordination chemistry: The potential role of transition-metal complexes in the chemical evolution  

NASA Technical Reports Server (NTRS)

In approaching the extremely involved and complex problem of the origin of life, consideration of the coordination chemistry appeared not only as a possibility but as a necessity. The first model experiments appear to be promising because of prebiotic-type synthesis by means of transition-metal complexes. It is especially significant that in some instances various types of vitally important substances (nucleic bases, amino acids) are formed simultaneously. There is ground to hope that systematic studies in this field will clarify the role of transition-metal complexes in the organizatorial phase of chemical evolution. It is obvious that researchers working in the fields of the chemistry of cyano and carbonyl complexes, and of the catalytic effect of transition-metal complexes are best suited to study these aspects of the attractive and interesting problem of the origin of life.

Beck, M.

1979-01-01

450

Recent advances in transition metal-catalyzed enantioselective hydrogenation of unprotected enamines.  

PubMed

Transition metal-catalyzed enantioselective hydrogenation of enamines is undoubtedly a useful and environment-friendly method for the preparation of optically pure chiral amines and amine derivatives. Over the last few decades, the use of transition metal catalysts containing chiral phosphorus or phosphine-oxazoline ligands attracted much attention for the hydrogenation of unprotected enamines. A number of efficient chiral catalysts have been developed, and some of them have shown high potential for the application in the synthesis of optical chiral amines in both laboratory and industry. This tutorial review focuses on the contributions concerning the transition metal-catalyzed enantioselective hydrogenation of unprotected enamines for the synthesis of chiral amines and amine derivatives. PMID:22509499

Xie, Jian-Hua; Zhu, Shou-Fei; Zhou, Qi-Lin

2012-06-01

451

Criticality of the metal-topological insulator transition driven by disorder  

NASA Astrophysics Data System (ADS)

Employing scaling analysis of the localization length, we deduce the critical exponent of the metal-topological insulator transitions induced by disorder. The obtained exponent ?˜2.7 shows no conspicuous deviation from the value established for metal-ordinary insulator transitions in systems of the symplectic class. We investigate the topological phase diagram upon carrier doping to reveal the nature of the so-called topological Anderson insulator (TAI) region. The critical exponent of the metal-TAI transition is also first estimated, shown to be undistinguishable from the above value within the numerical error. By symmetry considerations we determine the explicit form of Rashba spin-orbit coupling in systems of C4v point group symmetry.

Yamakage, Ai; Nomura, Kentaro; Imura, Ken-Ichiro; Kuramoto, Yoshio

2013-05-01

452

Model of Ultrafast Intersystem Crossing in Photoexcited Transition-Metal Organic Compounds  

NASA Astrophysics Data System (ADS)

The mechanism behind fast intersystem crossing in transition-metal complexes is shown to be a result of the dephasing of the photoexcited state to the phonon continuum of a different state with a significantly different transition metal-ligand distance. The coupling is a result of the spin-orbit interaction causing a change in the local moment. A recurrence to the initial state is prevented by the damping of the phonon oscillation. The decay time is faster than the oscillation frequency of the transition metal-ligand stretch mode, in agreement with experiment. For energies above the region where the strongest coupling occurs, a slower “leakage-type” decay is observed. If the photoexcited state is lower in energy than the state it couples to, then there is no decay.

van Veenendaal, Michel; Chang, Jun; Fedro, A. J.

2010-02-01

453

Valence-dependent analytic bond-order potential for transition metals  

NASA Astrophysics Data System (ADS)

An analytic interatomic bond-order potential is derived that depends explicitly on the valence of the transition-metal element. It generalizes the second-moment Finnis-Sinclair and fourth-moment Carlsson potentials to include higher moments. We find that the sixth-moment approximation predicts not only the structural trend from hcp?bcc?hcp?fcc that is observed across the nonmagnetic 4d and 5d transition-metal series, but also the different ferromagnetic moments of the bcc, fcc, and hcp phases of the 3d transition-metal iron. An analytic expression for the force is obtained and proved to converge to the Hellmann-Feynman force as higher moments are included.

Drautz, Ralf; Pettifor, D. G.

2006-11-01

454

Band gap tuning in transition metal oxides by site-specific substitution  

DOEpatents

A transition metal oxide insulator composition having a tuned band gap includes a transition metal oxide having a perovskite or a perovskite-like crystalline structure. The transition metal oxide includes at least one first element selected form the group of Bi, Ca, Ba, Sr, Li, Na, Mg, K, Pb, and Pr; and at least one second element selected from the group of Ti, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, and Pt. At least one correlated insulator is integrated into the crystalline structure, including REMO.sub.3, wherein RE is at least one Rare Earth element, and wherein M is at least one element selected from the group of Co, V, Cr, Ni, Mn, and Fe. The composition is characterized by a band gap of less of 4.5 eV.

Lee, Ho Nyung; Chisholm, Jr., Matthew F; Jellison, Jr., Gerald Earle; Singh, David J; Choi, Woo Seok

2013-12-24

455

On the Metal-Insulator Transition in a Two-Dimensional Hubbard Model  

NASA Astrophysics Data System (ADS)

Antiferromagnetic and metal to insulator transitions in the ground state of a two dimensional half-filled Hubbard model with the nearest neighbor and the second nearest neighbor transfers (-t and t?, respectively) are studied by using the Hartree-Fock approximation. The results indicate that the phase diagram depends on the band structure or t?/t ratio; a discontinuous transition between the paramagnetic metallic (PM) and the antiferromagnetic insulator (AI) phases takes place for t?/t=0.2, while continuous transitions from PM to AM (antiferromagnetic metal) and then to AI phases take place for t?/t=0.5. The results are compared with those of the Gutzwiller approximation and discussed in the light of the other existing theories. The general importance of considering antiferromagnetism in this problem is emphasized.

Kondo, Hisashi; Moriya, Tôru

1996-08-01

456

Stepwise addition of difluorocarbene to a transition metal centre.  

PubMed

The Ruppert-Prakash reagent (Me3SiCF3) is used to introduce difluorocarbene (CF2) and tetrafluoroethylene (TFE) ligands to cobalt(I) metal centres, whereby the TFE ligand is generated via [2+1] cycloaddition between [Co]=CF2 and CF2. PMID:24322965

Lee, Graham M; Harrison, Daniel J; Korobkov, Ilia; Baker, R Tom

2014-02-01

457

Effect of nominal substitution of transition metals for excess Fe in Fe1+xSe superconductor  

NASA Astrophysics Data System (ADS)

Taking cue from the increase in the superconducting transition temperature (Tc) of Fe1+xSe via nominal (2 wt%) substitution of Cr instead of excess Fe, we have now extended our study with nominal substitution (?5 wt%) with other transition metals (Ni, Co, Fe, Mn, Cr, V and Ti) in place of excess iron. The Tc is found to increase (maximum ~11 K) or get suppressed depending on the substituted transition metal. Our studies indicate that the superconducting transition temperature depends on ionic size of the transition metal and amount of hexagonal phase present as impurity.

Yadav, Anil K.; Sanchela, Anup V.; Thakur, Ajay D.; Tomy, C. V.

2015-01-01

458

Structure, Bonding, and Catalytic Activity of Monodisperse, Transition-Metal-Substituted CeO2 Nanoparticles.  

PubMed

We present a simple and generalizable synthetic route toward phase-pure, monodisperse transition-metal-substituted ceria nanoparticles (M0.1Ce0.9O2-x, M = Mn, Fe, Co, Ni, Cu). The solution-based pyrolysis of a series of heterobimetallic Schiff base complexes ensures a rigorous control of the size, morphology and composition of 3 nm M0.1Ce0.9O2-x crystallites for CO oxidation catalysis and other applications. X-ray absorption spectroscopy confirms the dispersion of aliovalent (M(3+) and M(2+)) transition metal ions into the ceria matrix without the formation of any bulk transition metal oxide phases, while steady-state CO oxidation catalysis reveals an order of magnitude increase in catalytic activity with copper substitution. Density functional calculations of model slabs of these compounds confirm the stabilization of M(3+) and M(2+) in the lattice of CeO2. These results highlight the role of the host CeO2 lattice in stabilizing high oxidation states of aliovalent transition metal dopants that ordinarily would be intractable, such as Cu(3+), as well as demonstrating a rational approach to catalyst design. The current work demonstrates, for the first time, a generalizable approach for the preparation of transition-metal-substituted CeO2 for a broad range of transition metals with unparalleled synthetic control and illustrates that Cu(3+) is implicated in the mechanism for CO oxidation on CuO-CeO2 catalysts. PMID:25406101

Elias, Joseph S; Risch, Marcel; Giordano, Livia; Mansour, Azzam N; Shao-Horn, Yang

2014-12-10

459

On the behavior of Bronsted-Evans-Polanyi Relations for Transition Metal Oxides  

SciTech Connect

Versatile Broensted-Evans-Polanyi (BEP) relations are found from density functional theory for a wide range of transition metal oxides including rutiles and perovskites. For oxides, the relation depends on the type of oxide, the active site and the dissociating molecule. The slope of the BEP relation is strongly coupled to the adsorbate geometry in the transition state. If it is final state-like the dissociative chemisorption energy can be considered as a descriptor for the dissociation. If it is initial state-like, on the other hand, the dissociative chemisorption energy is not suitable as descriptor for the dissociation. Dissociation of molecules with strong intramolecular bonds belong to the former and molecules with weak intramolecular bonds to the latter group. We show, for the prototype system La-perovskites, that there is a 'cyclic' behavior in the transition state characteristics upon change of the active transition metal of the oxide.

Vojvodic, Aleksandra

2011-08-22

460

Photoresponse study on transition metal (Co, Ni, Mn) doped ZnO thin films  

NASA Astrophysics Data System (ADS)

The photoresponse characteristics of chemical solution deposited, undoped and transition metal doped zinc oxide thin films have been studied. The x-ray diffraction studies confirm the formation of the crystalline ZnO phase. The FESEM analysis of the deposited films indicates a granular surface with grains of hexagonal facet morphology. The photoluminescence spectra of the deposited films show a strong violet-blue-green emission band. The devices for photoresponse study were made by in-plane Ag contacts on the films, to form metal-semiconductor-metal configuration. The current-voltage characteristics of our transition metal doped ZnO films under illumination reveals the suitability of our thin films in visible-infra red light sensing applications.

Rajalakshmi, R.; Angappane, S.

2013-02-01

461

Density of states scaling at the semimetal to metal transition in three dimensional topological insulators.  

PubMed

The quantum phase transition between the three dimensional Dirac semimetal and the diffusive metal can be induced by increasing disorder. Taking the system of a disordered Z2 topological insulator as an important example, we compute the single particle density of states by the kernel polynomial method. We focus on three regions: the Dirac semimetal at the phase boundary between two topologically distinct phases, the tricritical point of the two topological insulator phases and the diffusive metal, and the diffusive metal lying at strong disorder. The density of states obeys a novel single parameter scaling, collapsing onto two branches of a universal scaling function, which correspond to the Dirac semimetal and the diffusive metal. The diverging length scale critical exponent ? and the dynamical critical exponent z are estimated, and found to differ significantly from those for the conventional Anderson transition. Critical behavior of experimentally observable quantities near and at the tricritical point is also discussed. PMID:24483912

Kobayashi, Koji; Ohtsuki, Tomi; Imura, Ken-Ichiro; Herbut, Igor F

2014-01-10

462

Density of States Scaling at the Semimetal to Metal Transition in Three Dimensional Topological Insulators  

NASA Astrophysics Data System (ADS)

The quantum phase transition between the three dimensional Dirac semimetal and the diffusive metal can be induced by increasing disorder. Taking the system of a disordered Z2 topological insulator as an important example, we compute the single particle density of states by the kernel polynomial method. We focus on three regions: the Dirac semimetal at the phase boundary between two topologically distinct phases, the tricritical point of the two topological insulator phases and the diffusive metal, and the diffusive metal lying at strong disorder. The density of states obeys a novel single parameter scaling, collapsing onto two branches of a universal scaling function, which correspond to the Dirac semimetal and the diffusive metal. The diverging length scale critical exponent ? and the dynamical critical exponent z are estimated, and found to differ significantly from those for the conventional Anderson transition. Critical behavior of experimentally observable quantities near and at the tricritical point is also discussed.

Kobayashi, Koji; Ohtsuki, Tomi; Imura, Ken-Ichiro; Herbut, Igor F.

2014-01-01

463

Optical conductivity of V4O7 across its metal-insulator transition  

NASA Astrophysics Data System (ADS)

The optical properties of a V4O7 single crystal have been investigated from the high-temperature metallic phase down to the low-temperature antiferromagnetic insulating phase. The temperature-dependent behavior of the optical conductivity across the metal-to-insulator (MIT) transition can be explained in a polaronic scenario. Charge carriers form strongly localized polarons in the insulating phase, as suggested by a far-infrared charge gap abruptly opening at TMIT ? 237 K. In the metallic phase, instead, the presence of a Drude term is indicative of fairly delocalized charges with a moderately renormalized mass m*?5me. The electronic spectral weight is almost recovered on an energy scale of 1 eV, which is much narrower than in the VO2 and V2O3 cases. Those findings suggest that electron-lattice interaction rather than electronic correlation is the driving force for the V4O7 metal-insulator transition.

Lo Vecchio, I.; Autore, M.; D'Apuzzo, F.; Giorgianni, F.; Perucchi, A.; Schade, U.; Andreev, V. N.; Klimov, V. A.; Lupi, S.

2014-09-01

464

How does quantum confinement influence the electronic structure of transition metal sulfides TmS2  

E-print Network

Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its size to a monolayer, MoS2 undergoes a transition to the direct band semiconductor. We support this experimental observation by first principles calculations and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale. We further studied the properties of related TmS2 nanolayers (Tm = W, Nb, Re) and show that the isotopological WS2 exhibits similar electronic properties, while NbS2 and ReS2 remain metallic independent on size.

Kuc, Agnieszka; Heine, Thomas

2011-01-01

465

Lattice vibrations and electronic transitions in the rare-earth metals: yttrium, gadolinium and lutetium.  

PubMed

Lattice vibrations in high-pressure phases of Y, Gd and Lu were studied by Raman spectroscopy. The observed phonon frequencies decrease towards the transitions to the dhcp and fcc phases. There is evidence that the entire structural sequence [Formula: see text] under pressure for the individual regular rare-earth metals and along the lanthanide series at ambient pressure involve softening of certain acoustic and optical phonon modes and of the elastic shear modulus C(44). Comparison is made to transitions between close-packed lattices in other metals, and possible correlations to s-d electron transfer are discussed. PMID:21690667

Olijnyk, Helmut

2005-01-12

466

Low Critical Concentration of Metal--Insulator Transition of Vanadium Doped Amorphous Boron  

Microsoft Academic Search

The metal--insulator transition (MIT) of V- and Zr-doped amorphous boron films (VxB100-x, ZrxB100-x) was studied. The temperature dependences of the electrical conductivity, the X-ray diffraction patterns and the X-ray absorption fine structures were measured for the samples. The compositional boundary of the metal--insulator transition is found between x = 2.9 and 3.7 in VxB100-x, and between x = 11 and

Kenji Tanabe; Kohei Soga; Shizuka Hosoi; Kazuaki Osumi; Hideshi Yamaguchi; Tomoya Uruga; Kaoru Kimura

2011-01-01

467

Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium.  

PubMed

We have performed quantum molecular dynamics simulations for dense helium to study the nonmetal-to-metal transition at high pressures. We present new results for the equation of state and the Hugoniot curve in the warm dense matter region. The optical conductivity is calculated via the Kubo-Greenwood formula from which the dc conductivity is derived. The nonmetal-to-metal transition is identified at about 1 g/cm(3). We compare with experimental results as well as with other theoretical approaches, especially with predictions of chemical models. PMID:17677615

Kietzmann, André; Holst, Bastian; Redmer, Ronald; Desjarlais, Michael P; Mattsson, Thomas R

2007-05-11

468

Metal Insulator Transition in a Continuum System and its Description within a Lattice Model  

NASA Astrophysics Data System (ADS)

The metal insulator transition in the half-filled Hubbard model with repulsive interactions describes a physical situation where the occupation of some localized orbitals changes during the transition. It is implicitly assumed that the orbitals themselves are suitable for describing both the metallic and insulating phases. We present a simple system where this assumption is not valid: a two-dimensional electron gas with an applied sinusoidal potential. If the applied potential is strong enough, it induces a metal insulator transition, and during this transition, the character of the electronic orbitals changes drastically. It is shown that this transition is not well reproduced within a simple Hubbard model. A simple Hubbard model cannot describe, even qualitatively, the energetics of both the insulating and metallic phases as a function of the external potential correctly for the same set of parameters. Only an extended Hubbard model can capture the essential physics, and we discuss an algorithm for extracting unique parameters for such a model from Hartree-Fock calculations on the continuum system.

Engel, Gerhard E.; Martin, Richard M.; Pickett, Warren E.

1997-03-01

469

Surface-enhanced Raman scattering from transition metals with special surface morphology and nanoparticle shape.  

PubMed

This discussion focuses on our recent approaches at aiming to optimize surface-enhanced Raman scattering (SERS) activity for transition metals (group VIII B elements), by intentionally fabricating desired surface nanostructures or synthesizing nanoparticles. The SERS activity of transition metals critically depends on the surface morphology of electrodes and on size, shape and aggregation form of nanoparticles. A correct surface roughening procedure for transition-metal electrodes is indispensable for fabricating cauliflower-like nanostructures that show a higher SERS activity. Two more methods have been explored to synthesize nanoparticles, i.e., cubic nanoparticles and gold-core palladium-shell nanostructures, respectively. Their SERS activities are considerably higher than those of normal spherical mono-metallic nanoparticles. To explain these observations, a preliminary theoretical calculation, using the three-dimensional finite difference time domain (3D-FDTD) method, was performed to evaluate the local electromagnetic field on transition metal surfaces. The result is in good agreement with the experimental data. PMID:16833114

Tian, Zhong-Qun; Yang, Zhi-Lin; Ren, Bin; Li, Jian-Feng; Zhang, Yong; Lin, Xu-Feng; Hu, Jia-Wen; Wu, De-Yin

2006-01-01

470

Metal insulator transitions in perovskite SrIrO3 thin films  

NASA Astrophysics Data System (ADS)

Understanding of metal insulator transitions in a strongly correlated system, driven by Anderson localization (disorder) and/or Mott localization (correlation), is a long standing problem in condensed matter physics. The prevailing fundamental question would be how these two mechanisms contrive to accomplish emergent anomalous behaviors. Here, we have grown high quality perovskite SrIrO3 thin films, containing a strong spin orbit coupled 5d element Ir, on various substrates such as GdScO3 (110), DyScO3 (110), SrTiO3 (001), and NdGaO3 (110) with increasing lattice mismatch, in order to carry out a systematic study on the transport properties. We found that metal insulator transitions can be induced in this system; by either reducing thickness (on best lattice matched substrate) or changing degree of lattice strain (by lattice mismatch between film and substrates) of films. Surprisingly these two pathways seek two distinct types of metal insulator transitions; the former falls into disorder driven Anderson type whereas the latter turns out to be of unconventional Mott-Anderson type with the interplay of disorder and correlation. More interestingly, in the metallic phases of SrIrO3, unusual non-Fermi liquid characteristics emerge in resistivity as ?? ? T? with ? evolving from 4/5 to 1 to 3/2 with increasing lattice strain. We discuss theoretical implications of these phenomena to shed light on the metal insulator transitions.

Biswas, Abhijit; Kim, Ki-Seok; Jeong, Yoon Hee

<