Mosaic Phase Research Articles

Cooling rate dependence of charge density wave phases in 1T-TaS2 studied by scanning tunneling microscopy and x-ray diffraction

The role of dynamics in the charge density wave (CDW) phases of 1T-TaS2, especially in accessing metastable phases, is still under scrutiny. We investigated cooling rate dependence on low-temperature CDW phases in this material by scanning tunneling microscopy and x-ray diffraction. In the majority of cases, we found the typical low temperature commensurate CDW and identified no other differences between samples that were fast cooled vs slow cooled from room temperature. In rare cases (1/18 STM experiments and 1/30 XRD experiments), we found a multi-domain structure in the low temperature CDW. The domain structure and metallic behavior revealed by STM and STS measurements of this phase are in excellent agreement with the previously reported metallic mosaic CDW phase.

Observation of the metallic mosaic phase in 1T−TaS2 at equilibrium

The transition-metal dichalcogenide tantalum disulphide ($1\mathit{T}\text{\ensuremath{-}}\mathrm{TaS}{}_{2}$) hosts a commensurate charge density wave (CCDW) at temperatures below 165 K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable ``mosaic'' phase by means of either laser or voltage pulses, which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we observe the occurrence of such a metallic mosaic phase on the surface of $\mathrm{TaS}{}_{2}$ without prior pulse excitation over continuous areas larger than $100\ifmmode\times\else\texttimes\fi{}100\phantom{\rule{0.16em}{0ex}}\mathrm{nm}{}^{2}$ and macroscopic areas on the millimeter scale. We attribute the appearance of the mosaic phase to the presence of surface defects which cause the formation of the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behavior in the mosaic phase could be explained by local stacking differences of the top layer. Thus we provide a potential avenue to explain the origin of the pulse-induced mosaic phase.

Electronic states of domain walls in commensurate charge density wave ground state and mosaic phase in 1T -TaS2

Domain walls (DWs) in the charge-density-wave (CDW) Mott insulator 1T-TaS2 have unique localized states, which play an important role in exploring the electronic properties of the material. However, the electronic states in DWs in 1T-TaS2 have not been clearly understood, mostly due to the complex structures, phases, and interlayer stacking orders in the DW areas. Here, we explored the electronic states of DWs in the large-area CDW phase and mosaic phase of 1T-TaS2 by scanning tunneling spectroscopy. Due to the different densities of DWs, the electronic states of DWs show distinct features in these phases. In the large area CDW phase, both the domain and the DWs (DW1, DW2, DW4) have zero conductance at the Fermi level; while in the mosaic phase, they can be metallic or insulating depending on their environments. In areas with a high density of DWs, some electronic states were observed both on the DWs and within the domains, indicating delocalized states over the whole region. Our work contributes to further understanding of the interplay between CDW and electron correlations in 1T-TaS2.

Microstructure and Tribological Properties of Lubricating-Reinforcing Laser Cladding Composite Coating with the Ti2SC-Ti2Ni Mosaic Structure Phase

Lubricating-reinforcing composite coatings were successfully prepared on Ti6Al4V using laser-clad Ti6Al4V/Ni60/Ni-MoS2 mixed powders with different Ni-MoS2 contents (25, 35, and 45 wt.%), and their microstructure and tribological properties were studied. The reinforcing phase TiC, Ti2Ni, and the lubricating phase Ti2SC were in situ precipitated while Ti2SC and Ti2Ni formed a mosaic coherent structure within the above three coatings. In the 25 and 45 wt.% Ni-MoS2 coatings, the microstructure distribution uniformity of the coatings was not effectively improved by the Ti2SC-Ti2Ni mosaic structure phase due to the lower or higher content of Ti2SC. In the 35 wt.% Ni-MoS2 coating, the forming quality of the coating was the best due to an appropriate amount of the uniformly distributed Ti2SC-Ti2Ni mosaic structure phase. Furthermore, the microhardness of the coatings gradually decreased as the amount of Ni-MoS2 increased. In the 35 wt.% Ni-MoS2 coating, due to the uniformly and diffusely distributed Ti2SC-Ti2Ni mosaic structure phase, the stable lubricating-reinforcing mosaic structure transfer composite films were formed during the progress of the friction and wear tests, which led to the optimal worn surface evenness and quality, the anti-friction and the wear resistance properties compared with the Ti6Al4V, 25 and 45 wt.% Ni-MoS2 coating.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) Spectrographs

We describe the design and performance of the near-infrared (1.51–1.70 μm), fiber-fed, multi-object (300 fibers), high resolution (R = λ/Δλ ∼ 22,500) spectrograph built for the Apache Point Observatory Galactic Evolution Experiment (APOGEE). APOGEE is a survey of ∼105 red giant stars that systematically sampled all Milky Way populations (bulge, disk, and halo) to study the Galaxy’s chemical and kinematical history. It was part of the Sloan Digital Sky Survey III (SDSS-III) from 2011 to 2014 using the 2.5 m Sloan Foundation Telescope at Apache Point Observatory, New Mexico. The APOGEE-2 survey is now using the spectrograph as part of SDSS-IV, as well as a second spectrograph, a close copy of the first, operating at the 2.5 m du Pont Telescope at Las Campanas Observatory in Chile. Although several fiber-fed, multi-object, high resolution spectrographs have been built for visual wavelength spectroscopy, the APOGEE spectrograph is one of the first such instruments built for observations in the near-infrared. The instrument’s successful development was enabled by several key innovations, including a “gang connector” to allow simultaneous connections of 300 fibers; hermetically sealed feedthroughs to allow fibers to pass through the cryostat wall continuously; the first cryogenically deployed mosaic volume phase holographic grating; and a large refractive camera that includes mono-crystalline silicon and fused silica elements with diameters as large as ∼400 mm. This paper contains a comprehensive description of all aspects of the instrument including the fiber system, optics and opto-mechanics, detector arrays, mechanics and cryogenics, instrument control, calibration system, optical performance and stability, lessons learned, and design changes for the second instrument.

A metallic mosaic phase and the origin of Mott-insulating state in 1T-TaS2.

Electron–electron and electron–phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. In layered compound 1T-TaS2, the intricate interplay between the two generates a Mott-insulating ground state with a peculiar charge-density-wave (CDW) order. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses, and find that the new phase exhibits electronic structures entirely different from that of the original Mott ground state. The mosaic phase consists of nanometre-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layers underneath. We discover that the nature of the new phase is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in 1T-TaS2.

An STM study of desorption-induced thallium structures on the Si(111) surface

The scanning tunneling microscopy is used to study morphology of a Tl adlayer in various stages of Tl desorption from the Si(111) surface. Transition from the Si(111)/(1×1)-Tl structure through the (√3×√3)R30° mosaic phase to domains of metastable Si reconstructions is observed. Silicon substitutional atoms are found to be intrinsic to the (√3×√3)R30° structure. The temperature dependence of the amount of residual Tl atoms on the surface is successfully fitted by a model using the first order desorption. The same desorption energy of (2.1±0.3) eV and frequency prefactor 5×1014±2s−1 during all stages of the desorption are sufficient for the fitting. It is concluded that bonding of Tl in both (1×1) and (√3×√3) configurations is of the same nature.

Intra-atomic charge re-organization at the Pb–Si interface: Bonding mechanism at low coverage

Lead submonolayers on the Si(1 1 1) surface in ordered structures of symmetry ( 3 × 3 ) R 30 ° were studied with photoelectron diffraction and ab initio calculations. At 1/3 monolayer coverage Pb atoms flip between up and down positions. This fluctuating geometry is connected with the charge redistribution in the basic triangular bipyramid formed by an adsorbed Pb atom and its four closest Si neighbors. The dynamic balance is reflected in the switching of bipyramids from semiconductor to metal character as a result of the interaction between an unsaturated Pb atom in the T 4 position and a saturated Si atom beneath. Substitution of a Pb atom in the 1/3 monolayer coverage with a Si atom results in stabilisation of the neighboring Pb atoms in the up position which is characteristic for Pb atoms in the 1/6 monolayer mosaic phase.

Atom-by-Atom Chemical Coordination Effect Observed in Noncontact AFM Topography of Pb/Si(111)-(.RAD.3 * .RAD.3) Mosaic Phase

Using noncontact atomic force microscopy (NC-AFM), we analyzed the height distribution of individual atoms by atomically resolved topography of Pb/Si(111)-(√3 × √3) mosaic phase surfaces, discriminated Pb, and substituted Si adatoms atom-by-atom. This successfully revealed the atom-by-atom chemical coordination effect in the height distribution of individual atoms. We found that increasing the number of surrounding nearest-neighbor Si adatoms increases the height of Si adatoms but rapidly decreases the height of Pb adatoms. These coordination effects were qualitatively independent of the tip-sample surface distance. This result agreed with previous results on Sn/Si(111)-(√3 × √3) mosaic phase for only Si adatoms. Pb adatoms, however, showed a strong but opposite dependence on the number of surrounding Si adatoms, and the Sn adatom proved independent of the number of surrounding Si adatoms. Such differences in the chemical coordination in NC-AFM height distribution among the fourth period elements of Si, Sn, and Pb atoms will disclose to differences in charge transfer and related phenomena between intermixed heterogeneous atoms, due to differences in electronegativity and/or their metallic nature. [DOI: 10.1380/ejssnt.2008.79]

Two‐phase scale‐based reduction method for fulfilling monitoring service on mobile devices

AbstractOne of the general location‐based services (LBSs) is the monitoring of real‐time locations of moving objects. When the number of moving objects is large and the task of monitoring is carried out on mobile devices, the monitoring service suffers from constraints of screen size, computing speed, and network bandwidth. In the present paper, a two‐phase scale‐based reduction method (SRM) consisting of a zoom phase and a mosaic phase, is proposed to overcome these constraints. The zoom phase reduces the original monitoring area which, in turn, undergoes further reduction in the mosaic phase. The performance was measured with the use of two ratios: the reduction ratio (RRatio) and the transmission ratio (TRatio). From the experimental results, the lowest RRatio was 52%, i.e. almost half of the original data size was reduced. The lowest average TRatio was also 52% for the worst case, i.e. the entire original monitoring area was displayed on the mobile device. Moreover, the display time was shortened from 14.3 to 0.7 s. These results show that the use of the two‐phase SRM is practical and efficient when applied to the monitoring service on mobile devices. Copyright © 2006 John Wiley & Sons, Ltd.

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

Surface phase behavior of n-hexadecyl phosphate ( n-HDP) and its mixture with l-arginine (L-arg), which behaves as l-argininium cation (L-arg +) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at ⩽20 °C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at ⩾10 °C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the π – t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg +.

Non-contact atomic force microscopy study of the Sn/Si(1 1 1) mosaic phase

We have analyzed the height dependence of the Si and Sn atoms on the 1/6 monolayer (ML) Sn/Si(1 1 1)-(√3 × √3) R30° surface, also known as Sn/Si(1 1 1) mosaic phase, by means of non-contact atomic force microscopy (NC-AFM) technique. By preparing samples in which the Sn/Si(1 1 1) mosaic phase and the Si(1 1 1)-(7 × 7) surfaces coexist, and taking account of the proportion of bright and dim contrast atoms when comparing NC-AFM images of both surfaces, we have been able to discriminate between the two atom species forming the mosaic phase. Additionally, we have found a pronounced variation of the height of the Si adatoms with the number of first neighboring Sn adatoms in the Sn/Si(1 1 1) mosaic phase.

First-principles studies of structures and stabilities of Pb/Si(111)

Various structures of Pb/Si(111) from 1/6 ML to 4/3 ML are investigated using first-principles calculations. Our calculations for the 1/6-ML mosaic phase and 1/3-ML $\ensuremath{\beta}$-phase yield good agreement with the experimental results. The higher coverage range $1/3<\ensuremath{\theta}<4/3\mathrm{ML}$ has been controversial experimentally, especially close to 4/3 ML with several different phases being observed $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{7},$ hexagonal incommensurate, striped incommensurate, etc.). We compare the unit cell energies of different structures to deduce the energetically favorable ones. Structures consisting of very flat Pb overlayers have lower energy than missing-top-layer structures or structures with stacking faults. This is in very good agreement with experiments which measure high Pb mobility on the $\ensuremath{\alpha}$ phase.

Co-adsorption of Cu and Pb on the Si(1 1 1)–(7 × 7) surface: interface formation

The adsorption of Cu and Pb atoms on a Si(1 1 1)–7 × 7 surface was studied by means of scanning tunnelling microscopy (STM). After deposition of ⩽0.1 monolayer (ML) of copper on the Pb √3×√3 mosaic phase and a subsequent annealing up to ∼373 K, new objects of a hexagonal shape appear on a surface. They are scattered over the surface and localised next to the mosaic phase islands. The difference between atomically resolved images of filled and empty states suggests strong covalent bonding within a hexagon. Increasing the concentration of Cu atoms leads to an increasing number of hexagons and their agglomeration. Annealing of this surface at a higher temperature (>470 K) leads to the transformation of these hexagonal-like objects and their agglomeration into the pseudo-“5 × 5” structure, commonly observed for the Cu/Si(1 1 1)system. The absence of hexagons at very low Cu concentration demonstrates the presence of a long-range, attractive interaction among Cu atoms and their strong diffusion just above room temperature (RT).

Peculiar structure of X-ray standing wave lines of Si:As

X-ray standing wave (XSW) lines were measured for As-implanted Si wafers (Si:As) in order to investigate the annealing effect on local structure around As atoms. The observed XSW lines were spiral like and their patterns of an annealed and as-implanted Si:As samples were quite similar, though the EXAFS data of both samples showed different local structures. This result suggested that there existed stable mosaic crystalline phase in a certain region of Si wafer, which formed during As-implantation. The structural difference between the annealed and as-implanted Si:As samples was clarified by the XSW line measurements with highly angular resolution, resulting that the surfaces of the as-implanted Si:As sample were amorphous like and it crystallized by the annealing.

Quantized valence states of the Pb/Si(111) mosaic phase

The Pb/Si(111)-$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ mosaic phase, whose surface consists of $\ensuremath{\approx}\frac{1}{6}$ ML of Pb and $\frac{1}{6}$ ML of Si, has been studied by scanning tunneling spectroscopy. The top of the valence band consists of Pb ${6p}_{z}$ derived states, which are quantized into a series of about four equally spaced, narrow peaks. The sharpness and regular ordering of these states is unexpected considering that the surface is chemically disordered, with no long-range order. The existence of these states is attributed to correlations in the short-range order, and is consistent with published calculations of one-dimensional systems.

Contrasted electronic properties of Sn-adatom-based(3×3)R30°reconstructions on Si(111)

We have investigated the electronic structure of the two-dimensional solid solution ${\mathrm{Si}}_{x}{\mathrm{Sn}}_{1\ensuremath{-}x}/\mathrm{Si}(111)\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ at room temperature, with a particular emphasis on the empty states, using both global $[{k}_{//}$-resolved inverse photoemission spectroscopy (KRIPES)] and local probes (scanning tunneling microscopy and spectroscopy, STM and STS), as well as DFT-LDA calculations. This adatom overlayer with a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ symmetry shows drastic evolution with increasing Sn-adatom concentration, including a semiconductor to metal transition. The ${\mathrm{Si}}_{0.5}{\mathrm{Sn}}_{0.5}/\mathrm{Si}(111)\ensuremath{-}\sqrt{3}$ or mosaic phase has a single empty surface state localized at 0.56 eV above ${E}_{F}$ at $\overline{\ensuremath{\Gamma}}.$ With an overall bandwidth of $\ensuremath{\approx}0.15 \mathrm{eV},$ this ${\mathrm{sp}}_{z}$-type state localized on Si adatoms does not cross ${E}_{F}:$ the mosaic phase is semiconducting, with a bandgap between 0.3 and 0.5 eV. This phase is characterized by a large corrugation of 0.75 \AA{} with Sn adatoms higher than Si adatoms. In the Sn-rich limit ${\mathrm{Si}}_{x}{\mathrm{Sn}}_{1\ensuremath{-}x}/\mathrm{Si}(111)\ensuremath{-}\sqrt{3}$ with $x<~0.05,$ we follow an empty state ${U}_{1}^{\ensuremath{'}}$ throughout most of the surface Brillouin zone except near the $\overline{K}$ point where it clearly crosses the Fermi level. A second, empty surface state ${\mathrm{U}}_{2}^{\ensuremath{'}}$ is detected 1.67 eV above ${E}_{F}.$ Once correlation effects suggested by the small bandwidth of ${U}_{1}^{\ensuremath{'}}$ are adequately taken into account, we explain our KRIPES results in the framework of a dynamical fluctuations model as originating from an underlying $(3\ifmmode\times\else\texttimes\fi{}3)$ structure. Finally, results pertaining to intermediate Sn-adatom concentrations are interpreted in view of the two limiting cases.

Unusual Transition in a Two-Dimensional Condensed Phase to a Mosaic Texture

Both first- and second-order phase transitions have been studied in adsorbed monolayers of n-hexadecyl phosphate at the air−water interface by Brewster angle microscopy (BAM). An initial plateau region in the surface pressure−time (π−t) curve is caused by a first-order phase transition. This claim is supported by the coexistence of two surface phases which are observed by BAM. A second-order phase transition is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in surface pressure. This unusual transition from an isotropic to an anisotropic phase can be understood considering the intermediate nature of the former phase between liquid-expanded and liquid-condensed states.

Formation of an intermediate 3 × 3 phase from Pb on Si(111) at high temperature

Using high-resolution spot-profile-analysis low-energy electron diffraction (SPA-LEED) we have investigated low-coverage phases of Pb on Si(111) in the temperature range between 25 °C and 600 °C. After the annealing of 1/3 of a monolayer (ML) of Pb on Si(111) at 500 °C and cooling the sample down to room temperature, we observe a 3 × 3 phase beside the well known Pb/Si(111)(3)1/2 × (3)1/2R30° structure. The 3 × 3 phase is poorly ordered (domain size 20 Å-40 Å) compared to the (3)1/2 × (3)1/2 phase (domain size 1000 Å) as we conclude from profile analysis of the diffraction spots. The development of the 3 × 3 superstructure with increase of the annealing temperature was monitored and we observe a change of the domain size from 20 Å up to 40 Å upon annealing at 500 °C and 540 °C, respectively. After annealing at 580 °C the 3 × 3 phase has vanished and we detect only the (3)1/2 × (3)1/2 mosaic phase.

Optical transitions at the Si(111)-Pb(√3 × √3) mosaic phase

Optical transitions between surface state bands have been detected on the Si(111)-Pb(√3 × √3) mosaic phase by means of surface differential reflectivity (SDR) in the energy range between 1.3 and 3.8 eV. The SDR spectrum, obtained between the clean and the hydrogenated surface, presents a broad structure in this energy range with a maximum at 3.0 eV. The spectrum has been deconvolved into three Gaussian singlets at energies 1.8, 2.5 and 3.05 eV. These transitions have been, tentatively, assigned to different points in the surface Brillouin zone. No contribution from the hydrogenated surface is expected, as observed by angle resolved photoemission.