Nn Sites Research Articles

Tunneling interstitial impurity in iron-chalcogenide-based superconductors

A pronounced local in-gap zero-energy bound state (ZBS) has been observed by recent scanning tunneling microscopy (STM) experiments on the interstitial Fe impurity (IFI) and its nearest-neighboring (nn) sites in $\mathrm{FeTe_{0.5}Se_{0.5}}$ superconducting (SC) compound. By introducing a new impurity mechanism, the so-called tunneling impurity, and based on the Bogoliubove-de Gennes (BDG) equations we investigated the low-lying energy states of the IFI and the underlying Fe-plane. We found the peak of ZBS does not shift or split in a magnetic field as long as the tunneling parameter between IFI and the Fe-plane is sufficiently small and the Fe-plane is deep in the SC state. Our results are in good agreement with the experiments. We also predicted that modulation of spin density wave (SDW), or charge density wave (CDW) will suppress the intensity of the ZBS.

Central institutional review board review for an academic trial network.

Translating discoveries into therapeutics is often delayed by lengthy start-up periods for multicenter clinical trials. One cause of delay can be multiple institutional review board (IRB) reviews of the same protocol. When developing the Network for Excellence in Neuroscience Clinical Trials (NeuroNEXT; hereafter, NN), the National Institute of Neurological Disorders and Stroke (NINDS) established a central IRB (CIRB) based at Massachusetts General Hospital, the academic medical center that received the NN clinical coordinating center grant. The 25 NN sites, located at U.S. academic institutions, agreed to required CIRB use for NN trials. To delineate roles and establish legal relationships between the NN sites and the CIRB, the CIRB executed reliance agreements with the sites and their affiliates that hold federalwide assurance for the protection of human subjects (FWA); this took, on average, 84 days. The first NN protocol reviewed by the CIRB achieved full approval to allow participant enrollment within 56 days and went from grant award to the first patient visit in less than four months. The authors describe anticipated challenges related to institutional oversight responsibilities versus regulatory CIRB review as well as unanticipated challenges related to working with complex organizations that include multiple FWA-holding affiliates. The authors anticipate that CIRB use will decrease NN trial start-up time and thus promote efficient trial implementation. They plan to collect data on timelines and costs associated with CIRB use. The NINDS plans to promote CIRB use in future initiatives.

The effect of mineralogy on the mobility of heavy metals in mine tailings: a case study in the Samsanjeil mine, Korea

This study examined the chemical speciation and mobility of As and heavy metals in a tailings impoundment in Samsanjeil mine located in Gosung, Korea, as well as the factors affecting them. XRD, SEM, and 5-step sequential extraction were used to examine the samples at two sampling sites (NN and SN sites). The pH of the tailings decreased with increasing depth at the NN site (from 7.2 to 2.8), whereas no significant differences were observed at the SN site (8.1–8.8). The samples at the SN site showed a larger amount of calcite than those at the NN site, indicating that calcite plays an important role buffering the pH in the study sites. Jarosite was found only at the lower part of the NN site, where calcite was not found. The mineralogical observation of jarosite and calcite was also confirmed by SEM. The concentrations of As and heavy metals in the tailings were as follows: Cu > As > Zn > > Pb > Co > Cr > Ni > Cd. The total concentrations of Ni, Zn, Co, and Cd were higher at the SN site than those at the NN site. On the other hand, the concentrations of As and Cr existing as oxyanions were higher at the NN site, which can be explained by the mobility changes of those elements affected by pH variations. At the NN site, the fractions of heavy metals bound to the Fe/Mn oxides, except for As and Cr, decreased, and Cu, Zn, and Co showed an increasing fraction of exchangeable metals with increasing depth. This suggests that the pH and resulting surface charge of minerals, such as goethite and jarosite, are the dominant factors controlling the chemical speciation of metals. These results highlight the importance of mineralogy in controlling the mobility and possible bioavailability of heavy metals in tailings.

Use of single-grain geochemistry of cryptic tuffs and volcaniclastic sandstones improves the tephrostratigraphic framework of Olduvai Gorge, Tanzania

Single-grain geochemical composition of volcaniclastic sandstones can be a potential tool to improve correlations of mixed pyroclastic/epiclastic deposits. To test this, trachytic tuffs of the paleoanthropologically important FLK, FLK N, and FLK NN sites of Pleistocene Olduvai Gorge Bed I (Tanzania) are used as an established tephrostratigraphic framework against which to test volcaniclastic sandstone correlations. Fluvio-lacustrine sandstones and tuff samples were collected from eight archeological trenches between Tuffs IB and ID across a 500-m transect, including Leakey's famous Zinjanthropus (FLK) and OH 7/OH 8 (FLK NN) sites. A previously unknown, thin, fine, mineralogically unique, black trachyandesitic fallout ash was discovered below Tuff IC. Compositions of individual augite, feldspar and titanomagnetite grains from sandstones between Tuffs IB and IC reveal some IB-equivalent material, and a new compositional assemblage distinct from the sandwiching marker tuffs. Mineral compositions of the “tripartite” volcaniclastic sandstone between Tuffs IC and ID are similar to ID. Volcaniclastic sandstone grain fingerprints further refine correlations between fluvio-lacustrine sections within the area, providing support for proposed high-resolution stratigraphic reconstruction of the Zinjanthropus and OH 7/OH 8 land surfaces. This method might be applied to other sections where pyroclastic particles are admixed but distinct tuffs are not preserved.

Deep-sea nematode assemblages of the Clarion-Clipperton Nodule Province (Tropical North-Eastern Pacific)

The deep-sea nematode assemblages of the French mining claim area in the Clarion-Clipperton Nodule Province (the Northeastern Pacific Basin, 14°N, 130°W, depth 4,947–5,046 m) were studied at species, genus and family levels. Material was obtained using multicorers and box corers. In total, 2,174 individuals from 14 stations and 21 samples were examined. The nematode assemblages from the nodule-bearing (NB) part of the sampling area differed significantly from those at the nodule-free (NN) section. The average nematode density was 103 inds/10 cm2. The average nematode densities in the NB and NN parts were 69 inds/10 cm2 and 137 inds/10 cm2 respectively. Altogether, at least 325 putative species, 97 valid genera and 33 valid families were found. The real number of nematode species for the sampling area was estimated at about 510. The richest genera were Acantholaimus (33 putative species) and Thalassomonhystera (23 putative species). The proportions of juveniles, males and females among nematode individuals were about 44%, 23% and 33% respectively. About 2/3 of all nematodes were deposit-feeders and around 1/3 were epistrate-feeders. The nematode assemblage of the studied area may be considered to be a Theristus discolensis–Thalassomonhystera sp.3 assemblage with variation of the dominant species. The dominant species at the NN site were Theristus discolensis (12.2%) and Thalassomonhystera sp. 3 (5.3%). In contrast, at the NB site, Thalassomonhystera sp. 3 was most abundant (14.1%), followed by T. discolensis (6.2%). At genus level, Thalassomonhystera (17.9%), Theristus (13.0%) and Acantholaimus (12.8%) dominated at the NN site, whereas the abundance of Theristus was half of that (6.4%) at the NB site. The most dominant families at the NN and NB sites were Xyalidae (28.1% and 16.6%, respectively), Monhysteridae (17.9% and 25.3%) and Chromadoridae (16.2% and 22.5%).

Subsurface oxygen in monomer–dimer catalytic reaction: Influence of second and third nearest neighbourhood

The presence of subsurface species adds two extra steps to the usual four of simple Langmuir Hinshelwood mechanism of the monomer–dimer (NO–CO) surface reaction. In this model the influence of second and third nearest neighbouring (nn) sites on the phase diagram of the system is studied in detail by the use of Monte-Carlo simulation. It is seen that reaction between surface second nn sites has more dominant effect on the width of the reactive window as compared to that of subsurface second and third nn sites. In another model, NO is adsorbed in such a way that N takes surface site whereas O takes its subsurface first nn site. In this model the reaction of CO with subsurface second and third nn sites gives an interesting situation: The usual second-order phase transition is destroyed and consequently a very wide steady reactive window is observed.

Electronic structure and magnetic coupling in sodium electro sodalite: All-electron density functional calculations

Spin density functional calculations employing the full potential linearized augmented plane wave method (FLAPW) are performed on the periodic structure of sodium electro sodalite. The density functional adopted (PW91) includes gradient corrections for exchange and correlation. A body-centered cubic lattice of bare Na43+ clusters is found to be metallic and diamagnetic. The presence of the aluminosilicate framework makes it an antiferromagnetic material with a gap of about 0.1 eV between valence and conduction band. The antiferromagnetic state is more than 110–170 kJ mol−1 more stable than a ferromagnetic state. The Heisenberg exchange integrals between nearest and next nearest neighboring Na43+ clusters, Jnn and Jnnn, are derived from an extended Hückel tight-binding approach. The parameters of this Hamiltonian were chosen such that the density functional band structure is reproduced. The Heisenberg exchange integrals between nearest and next nearest neighboring Na43+ sites, Jnn and Jnnn, are negative, i.e., both nn and nnn sites are coupled antiferromagnetically. In absolute terms Jnnn is 0.6 meV. Estimates of Jnn are between 6.4 and 9.5 meV, the most likely value being 8.1±0.5 meV. Using these values for Jnn and Jnnn the molecular field approximation yields Weiss temperatures between −160 and −230 K. The most likely result, −200±10 K, fits well to the experimental value of about −200 K.

Electrical Study of Pr3+ Ion Environment in Ca1?xPrxF2+x Thin Films

The environment of Pr3+ ions and their spatial distribution in Ca1—xPrxF2+x thin films epitaxially grown on silicon substrates were studied by means of complex admittance spectroscopy (CAS) and thermally stimulated depolarization (TSD) studies. Contrary to the case of bulk single crystals, these techniques reveal only one type of praseodymium-fluoride compensating ion centers in the layers, whatever x varying from 0.001 to 0.10. These centers correspond to isolated substituted Pr3+ ions compensated by interstitial Fi— ions in nearest-neighbor (nn) sites of C4v symmetry. Moreover, it is demonstrated that for these praseodymium concentrations, the Fi— ions can be transferred between two neighboring centers. The corresponding dipolar characteristics show that the nn pairs are homogeneously distributed into the bulk of the layers. These results are of great interest. They indicate that energy transfers between excited rare-earth ions, which arise in bulk single crystals for high concentrations, must be considerably reduced in thin films. Consequently the self-quenching phenomena, which rapidly annihilate the luminescence, must be minimized.

Study of Ca 1 −xPr xF 2+ x solid solution thin films grown on silicon substrates

Ca 1 −x Pr xF 2+ x thin films, x varying from 0.01 to 0.10, were grown on Si (100) substrates by sublimation under ultra-high vacuum of solid solution powders obtained from high purity CaF 2 and PrF 3 initial powders. First, the synthesis of the bulk solid solutions was controlled using X-ray diffraction and Raman spectroscopy. A complete transformation seems to be obtained by heating for 8 hours under vacuum the mixed initial powders at a temperature of 1400 K. However, for x=0.10, a Rutherford backscattering (RBS) of α particles study shows that to obtain homogeneous thin films a pre-evaporation of the product at 1500 K during approximately 30 min is necessary. After such a treatment, the films are of the desired composition and the Pr 3+ ions are uniformly distributed within the layers. On the other hand, for the lower praseodymium contents, the alloy produced by heating the initial powders at 1400 K sublimes congruently. Moreover, in this case a RBS study in channeling conditions shows that the films are of epitaxial quality and a photoluminescence study shows that the interstitial fluoride ions, which balance the rare earth trivalent ions in the matrix, are preferentially located in nn (nearest neighbor) sites, of C 4v symmetry (tetragonal).

Diffusion and relaxations in liquid and amorphous metals

Abstract In molecular dynamics (MD) simulations of amorphous solids two distinct types of jump processes have been observed: collective jumps where a single atom moves only a fraction of the interatomic distance, and jumps where single atoms move over interatomic distances. Different jumps can interact via their long range displacement fields. Compared to vibrations the effects are reduced by temperature. In MD of undercooled liquid Zr67Cu33, two processes contribute to diffusion, resembling the two types of jumps in the solid. Additionally to a more ftow-like motion especially the Cu atoms jump over nearest-neighbour distances. The latter process dominates, at least for Cu, at temperatures sufficiently above the glass transition. With decreasing temperature these jumps over atomic distances become increasingly reversible and their contribution to long range diffusion diminishes. In conclusion, we have identified two distinct jump processes contributing to relaxation and diffusion. Below 0.5T g we observe only collective jumps of groups of atoms, typically along close-packed chains. In these jumps each single atom moves only a fraction of the NN distance and no inherent length scale is involved. At higher temperatures in the binary metal additional jumps of the smaller atoms (Cu) into NN sites are found. Judging from experiment these might be related to ageing. The relaxations in the glass are by local jumps. Similar to the vibrations, one has, due to the long range displacement fields, interaction effects between single relaxations. These are directly observed at low temperatures and will contribute to the increase with temperature of jump length and atomic participation. In the under-cooled melt of CuZr we identify for both species a flow-like motion corresponding to the collective jumps in the glass. At the onset of cooling diffusion of Cu is dominated by jumps into NN sites. These jumps become less effective upon cooling to T g due to a strong increase of jump-reversal. One can speculate that at T g the collective motion dominates for both species.

Simulation of oxygen desorption from Pt(111)

Using the Monte Carlo technique, we simulate the kinetics of temperature-programmed associative desorption (TPD) of oxygen atoms from Pt(111) at coverages below a quarter of the monolayer. Oxygen atoms are assumed to be adsorbed on a triangular lattice of fcc sites. Nearest-neighbour (nn) O–O lateral interactions, ϵ 1, are considered to be strongly repulsive so that there are no occupied nn sites. The second- and third-nn interactions, ϵ 2≥0 and ϵ 3⩽0 (positive values correspond to repulsion), are chosen to reproduce the p(2×2) ordering of oxygen atoms. Comparison between the simulations and experimental TPD spectra for the O/Pt(111) system is supporting repulsive O–O interactions. More specifically, the TPD spectra are hard to reconcile with the assumption that the O–O lateral interactions are partly attractive and that this attraction is sufficiently strong to stabilize the p(2×2) islands at low coverages and relatively high temperatures (∼400 K). The consequence of this finding for interpretation of available experimental data on oxygen diffusion on Pt(111) is discussed in detail.

Electrical characterization of Ca1−xErxF2+x luminescent thin films

Ca 1−x Er x F 2+x thin films, epitaxially grown on silicon substrates, present a 1.53 μm infrared luminescence line, which gives them an evident interest for optical communications. At room temperature, using an argon laser as the excitation source, the maximum of this emission is obtained in thin films for x=0.16, when it is observed for very low erbium contents in bulk CaF2:Er3+ single crystals. Thus, we can think that the Er3+ ion environment, which governs the self-quenching phenomena, differs in thin films from that in bulk single crystals. In this paper, the nature of the erbium doping centers and their spatial distribution into the host material are studied versus x by using complex admittance and thermally stimulated depolarization techniques. In thin films, it is shown that for x⩽0.01, the luminescent centers correspond to isolated Er3+–Fi− ion pairs in nn sites, which behave like dipoles, D1. Their activation energy is 0.68 eV. From x=0.01 to x=0.05 clustering begins to be observed and leads when x>0.05, to the formation of extended clusters, usually called D2a in the literature. So, the clustering process in thin films appears only for concentrations two orders of magnitude higher than in single crystals. Moreover, it is shown that, whatever x varying from 0.001 to 0.20, charges can be transferred between pairs or clusters. Then, the activation energy depends on the mean distance between these defects, and thus, on the erbium content: the dielectric response corresponds to a homogeneous distribution of the pairs or clusters into the bulk of the host matrix.

Hopping rates of H(D) bound to heavy interstitials in Ta as investigated by mechanical spectroscopy at MHz frequencies

The stress-induced reorientation of O(N)–H(D) pairs has been investigated at MHz frequencies in Ta by propagating longitudinal ultrasonic pulses along a <111> crystal lattice direction. The resulting anelastic relaxation is found to be composed of two processes which are attributed to two distinct configurations of the pair. A comparison of the transition rates of free and trapped H(D) atoms in Nb and Ta suggests that both processes observed in Ta involve transitions of H(D) atoms between equivalent nn sites, which are probably slightly displaced from normal tetrahedral sites (dis-1T↔dis-1T transitions). In Nb bound H(D) atoms should be in a state delocalized over two equivalent sites, probably slightly displaced from normal tetrahedral sites (dis-2T state), and the reorientation in this metal would take place via (dis-2T↔dis-2T) transitions.

Site-bond-correlated models for the disordered magnets: Application to KNipMg1−pF3 (abstract)

Site-bond-correlated (SBC) dilution picture has been successfully proposed to account for the striking differences experimentally observed by NMR measurements between random diluted magnet KNipMg1−pF3 and the isostructural compound KMnpMg1−pF3 (see for instance Ref. 1, and references therein). In the SBC picture, the coupling between two nearest-neighbors (nn) magnetic atoms is assumed to be dependent upon the random occupancy of the other nn sites. In this article we study the D-vector model disordered in accordance with the SBC picture in a d-dimensional hypercubic lattice within the framework of the mean field renormalization group. The study is based on two different models (denoted by A and B) for the exchange interactions, in which the random occupancy of the other nn sites is measured by means of a phenomenological parameter α. The critical boundaries of the ferromagnetic and the spin-glass phases with the paramagnetic one, are obtained for various values of the parameter α. The study reveals the existence of an interesting universal relation between the initial slope in the limiting correlated (α=0) and uncorrelated (α=1) cases, at p=1. It is shown that the D-vector classical model in the SBC version with inclusion of the relevant next-nearest-neighbors exchange interaction parameter, exhibits excellent agreement with the experimental data available for the compound KNipMg1−pF3 with α=0.145

Optical/RF double resonances in YLiF 4: Ho 3+

In this paper, double resonance signals associated with CW spectral holeburning measurements of unintentional Ho 3+ impurities in crystals of YLiF 4 are reported. The double resonance signals are detected in the microwave and radiofrequency region. The observed microwave resonances are at 4.460(1) and 4.473(1) GHz and attributed to ground state hyperfine splittings. Two radiofrequency resonances at 29.85(8) and 36.86(8) MHz are also detected and assigned to superhyperfine transitions of the two inequivalent fluorine nn sites. This assignment is supported by Zeeman measurements.

Relaxation modes of fluoride interstitials in gadolinium-doped SrF2.

The interstitial fluoride (${\mathrm{F}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}}$) of a simple ${\mathit{M}}^{3+}$-${\mathrm{F}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}}$ dipole in fluorite lattices can occupy any of six nearest-neighbor (nn) sites or eight next-nearest-neighbor (nnn) sites. Previous determinations of the relaxation modes of this center have included only nn-nn, nn-nnn, and nnn-nn jumps, ignoring jumps between adjacent nnn sites. We have solved the more general normal-mode problem allowing jumps between all adjacent sites of either species. This model allows comparison of the time constant of dipolar relaxation and that of approach to equilibrium under all assumptions about the relative magnitude of the rates of the four different possible jumps between adjacent sites. We have measured the time constant and activation enthalpy for dipolar relaxation by thermally stimulated depolarization and for the approach to equilibrium by electron paramagnetic resonance (EPR) below 200 K, and have found similar values for these two processes. We have also determined the relative populations of these sites over a broad range of temperature (100--310 K) from EPR data. Populations of the nn and nnn sites reach equilibrium with each other quickly at temperatures above 200 K; population changes are quenched at lower temperatures. The time constant for approach to equilibrium is nearly the same as that for dipolar relaxation at temperatures between 179 and 200 K, indicating the nnn-nnn jumps are unlikely below 200 K. Other work suggests that above room temperature alternate relaxation modes must be important.

Magnetism in bcc Fe-Al alloy

The magnetic moment Μ, hyperfine fieldBhf and isomer shiftIS at the Fe site in bcc Fe-Al alloys were calculated from first principle. Contrary to the belief that Al atoms reside in the iron lattice as magnetic holes, it was found that the local magnetic moment of Fe is decreased when Al is at a nearest neighbour site (NN), while it increases if Al is at a next-nearest neighbour site (NNN). Consequently, the averageΜ per Fe atom was found to be, initially, independent of Al content. Assuming a linear dependence ofΜ Fe on the number of Al atoms at NN and NNN sites, we calculated the averageΜ, which was found to agree with experimental results of cold worked alloys for disordered Fe-Al alloy. On the other hand, antiferromagnetic coupling appears in the CsCl ordered structures. The averageBhf was also calculated and compared to experimental data and the trends in theIS are considered.

Electric field gradient for muonium and monovacancies in Cu and Al

The electric field gradient (EFG) and asymmetry parameter (η) at the first nearest neighbour (1NN) site of muonium (μ +) and at the first four NN sites of vacancies in Cu and Al are calculated. The defect induced charge density, used to evaluate the valence effect EFG, is calculated self-consistently in the density functional formalism. The formalism for the size effect EFG for μ + at the octahedral site in the fcc lattice is developed in the elastic continuum model. For both the μ + and the vacancy the components of the strain field tensor are evaluated assuming the lattice of dressed point ions interacting through the screened Coulomb potential. The size effect EFG for μ + is found cylindrically symmetric at the 1NN and 2NN sites. The size effect EFG is found larger than the valence effect EFG at all the NN sites for Cu- and Al-vacancies and the same is obtained at the 1NN site for μ +. The valence effect EFG for an Al-vacancy is found to be relatively larger than for a Cu-vacancy while the contrary is concluded for the μ +. It is noted that η depends upon the relative phases of components of EFG and not on their absolute values independently. The calculated and the experimental values of EFG and η agree reasonably.

Dielectric loss of quartz crystals electrodiffused with either Na + or Li +

Dielectric relaxation techniques were used in order to better understand the behavior of defects that affect the frequency stability of α-quartz crystals. Crystals were studied that have been swept (electrodiffused) either with Na + or with Li +. The Na-swept samples showed two well-known loss peaks (called α and β) due to the Al-Na pair, corresponding to the Na + ion, respectively, in nn and nnn interstitial sites about a substitutional Al 3+ ion. Precise activation energies and preexponentials were obtained by comparison with analogous anelastic loss peaks. It was also found that the α:β equilibrium is frozen-in below 300 K and can be varied, by heat treatment. Values of the components of the dipole moment of the α and β pairs were determined, and absolute calibrations obtained for the two peaks. In the case of the Li-swept samples, however, there were no peaks present, from which it was concluded that for the Al-Li pair, the Li + ion sits on the twofold symmetry axis of the AlO 4 tetrahedron.

Electric field gradients in nonmagnetic dilute alloys of aluminum and copper

The electric field gradient (EFG) and the asymmetry parameter ($\ensuremath{\eta}$) are investigated at the first-, second, third-, and fourth-nearest-neighbor (NN) sites for nonmagnetic dilute alloys of Al and Cu. The impurity effective valency is calculated according to the Friedel criterion of bound states. The impurity-induced electronic charge density is calculated self-consistently in the Hohenberg-Kohn-Sham density-functional formalism and is used to evaluate the valence-effect contribution to the EFG. The size-effect contribution to the EFG is estimated in the elastic continuum model. Reliable values of the core-enhancement and Sternheimer antishielding factors are used. No size-strength parameter is introduced to explain the EFG's at the 2NN, 3NN, and 4NN sites. However, the EFG's at the 1NN site are explained with the help of a size-strength parameter ${C}^{\ensuremath{'}}$ which accounts for the deficiency of the elastic continuum model. The calculated values of EFG and are $\ensuremath{\eta}$ found to be in reasonable agreement with the experimental values for most of the alloys. In Al alloys the size-effect EFG is found to be dominant at the 1NN site and the valence-effect EFG is found to be dominant at the 2NN, 3NN, and 4NN sites. In Cu alloys the size-effect EFG is dominant at all the NN sites.