Surface Reconstructs Research Articles

Evidence for charge delocalization crossover in the quantum critical superconductor CeRhIn5.

The nature of charge degrees-of-freedom distinguishes scenarios for interpreting the character of a second order magnetic transition at zero temperature, that is, a magnetic quantum critical point (QCP). Heavy-fermion systems are prototypes of this paradigm, and in those, the relevant question is where, relative to a magnetic QCP, does the Kondo effect delocalize their f-electron degrees-of-freedom. Herein, we use pressure-dependent Hall measurements to identify a finite-temperature scale Eloc that signals a crossover from f-localized to f-delocalized character. As a function of pressure, Eloc(P) extrapolates smoothly to zero temperature at the antiferromagnetic QCP of CeRhIn5 where its Fermi surface reconstructs, hallmarks of Kondo-breakdown criticality that generates critical magnetic and charge fluctuations. In 4.4% Sn-doped CeRhIn5, however, Eloc(P) extrapolates into its magnetically ordered phase and is decoupled from the pressure-induced magnetic QCP, which implies a spin-density-wave (SDW) type of criticality that produces only critical fluctuations of the SDW order parameter. Our results demonstrate the importance of experimentally determining Eloc to characterize quantum criticality and the associated consequences for understanding the pairing mechanism of superconductivity that reaches a maximum Tc in both materials at their respective magnetic QCP.

Charge Localization Induced by Pentagons on Ge(110).

The Ge(110) surface reconstructs into ordered and disordered phases, in which the basic unit is a five-membered ring of Ge atoms (pentagon). The variety of surface reconstructions leads to a rich electronic density of states with several surface states. Using scanning tunneling microscopy and spectroscopy, we have identified the exact origins of these surface states and linked them to either the Ge pentagons or the underlying Ge-Ge bonds. We show that even moderate fluctuations in the positions of the Ge pentagonal units induce large variations in the local density of states. The local density of states modulates in a precise manner, following the geometrical constraints on tiling Ge pentagons. These geometry-correlated electronic states offer a vast configurational landscape that could provide new opportunities in data storage and computing applications.

2×2R45∘ surface reconstruction and electronic structure of BaSnO3 film

We studied surface and electronic structures of barium stannate (BaSnO$_3$) thin-film by low energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy (ARPES) techniques. BaSnO$_3$/Ba$_{0.96}$La$_{0.04}$SnO$_3$/SrTiO$_3$ (10 nm/100 nm/0.5 mm) samples were grown using pulsed-laser deposition (PLD) method and were \emph{ex-situ} transferred from PLD chamber to ultra-high vacuum (UHV) chambers for annealing, LEED and ARPES studies. UHV annealing starting from 300$^{\circ}$C up to 550$^{\circ}$C, followed by LEED and ARPES measurements show 1$\times$1 surfaces with non-dispersive energy-momentum bands. The 1$\times$1 surface reconstructs into a $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ one at the annealing temperature of 700$^{\circ}$C where the ARPES data shows clear dispersive bands with valence band maximum located around 3.3 eV below Fermi level. While the $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction is stable under further UHV annealing, it is reversed to 1$\times$1 surface by annealing the sample in 400 mTorr oxygen at 600$^{\circ}$C. Another UHV annealing at 600$^{\circ}$C followed by LEED and ARPES measurements, suggests that LEED $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction and ARPES dispersive bands are reproduced. Our results provide a better picture of electronic structure of BaSnO$_3$ surface and are suggestive of role of oxygen vacancies in the reversible $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1-10 μA·μm-1), and good field-effect transistor mobility (∼30 cm2·V-1·s-1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

Fermi surface reconstruction and anomalous low-temperature resistivity in electron-doped La2−xCexCuO4

We report $ab$-plane Hall-effect and magnetoresistivity measurements on $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{C}{\mathrm{e}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ thin films as a function of doping for magnetic fields up to 14 T and temperatures down to 1.8 K. A dramatic change in the low-temperature (1.8 K) normal-state Hall coefficient is found near a doping $\mathrm{Ce}=0.14$. This, along with a nonlinear Hall resistance as a function of magnetic field, suggests that the Fermi surface reconstructs at a critical doping of $\mathrm{Ce}=0.14$. A competing antiferromagnetic phase is the likely cause of this Fermi surface reconstruction. Low-temperature linear-in-$T$ resistivity is found at $\mathrm{Ce}=0.14$, but anomalously, also at higher doping. We compare our data with similar behavior found in hole-doped cuprates at a doping where the pseudogap ends.

Charge compensation by long-period reconstruction in strongly polar lithium niobate surfaces

The microscopic structure of the polar (000$\overline{1}$) and (0001) surfaces of lithium niobate is investigated by atomic-resolution frequency modulation atomic force microscopy and first-principles calculations. It is found that the surface reconstructs at annealing temperatures sufficiently high to drive off external adsorbates. In particular a ($\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$)$R$19.1${}^{\ensuremath{\circ}}$ reconstruction is found for the (000$\overline{1}$) surface. Density-functional theory calculations show that---apart from the $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})$---a series of adatom-induced surface reconstructions exist that lower the surface energy and reduce the surface charge.

Water Adsorption on the Reconstructed (001) Chalcopyrite Surfaces

The interaction of water molecule with the reconstructed (001) chalcopyrite surfaces has been investigated by means of density functional calculations. All of the calculations were performed using periodic boundary conditions with SIESTA code. The structural parameters were compared with those obtained through PWscf code in order to evaluate the pseudopotentials and numerical basis set developed for this work. Two different surfaces were studied, namely, sulfur-terminated, (001)-S, and metal-terminated, (001)-M. The (001)-S surface reconstructs, forming disulfide dimers with a bond length of 2.23 Å. The (001)-M surface reconstructs, reordering the metal atoms in order to form planes of metal atoms and interlaced sulfur atoms. Different adsorption sites for the water molecule were investigated. The dissociative mechanism of the water molecule has also been analyzed in detail. For the (001)-S surface, the water adsorption on the iron atom is the preferred mechanism. The dissociative mechanism leads to structures which are, at least, 13 kcal mol–1 higher in energy than the water adsorbed on iron atom. For the (001)-M surface, no minima in the potential energy surface were found, and the water molecule prefers to form a hydrogen bond with the sulfur atoms. The dissociative mechanism for the water adsorption on (001)-M surface is thermodynamically unfavorable. The metal-alloy-like structure underneath of the sulfur atoms and the unfavorable water adsorption indicate that the surface presents some hydrophobic character. The influence of the water molecule in the reconstruction of the (001) chalcopyrite surface and in its reactivity is discussed.

Formation and Properties of Responsive Siloxane‐Based Polymeric Surfaces with Tunable Surface Reconstruction Kinetics

AbstractHere, the formation of responsive polymeric materials with tunable response time is reported. These structures are fabricated by chemically modifying poly(vinylmethyl siloxane) (PVMS) networks with alkanethiols bearing a hydrophilic end‐group (COOH or OH). The response time is facilitated by the liquid nature of the PVMS backbone and increases with increase in length of the methylene spacer (CH2)n in the alkanethiol pendent group. While for n = 2 and 6, the surface reconstructs almost instantaneously, specimens with n = 11 resist reconstruction because of strong van der Waals forces, leading to the formation of semi‐crystalline regions. It is demonstrated that the responsive nature of PVMSS(CH2)11OH can be fine‐tuned by varying the temperature; it possesses a faster response at temperatures above the melting point of the S(CH2)11OH moiety.

Electronic structure of theC(111)surface: Solution by self-consistent many-body calculations

We present a first-principles calculation of the geometry and of the electronic structure for the C1112 1 surface. We find that this surface reconstructs with -bonded chains without any significant dimerization or buckling. At the DFT level, it appears semimetallic, in agreement with previous calculations, but in contrast to experimental evidence. Even the introduction of quasi-particle corrections within the usual GW scheme does not lead to the opening of the gap between surface states. Quasiparticle corrections to the surface band structure are hence calculated within a self-consistent GW scheme. A gap of about 1 eV is found between the surface states, thus finally solving the discrepancy between theory and experiment. The 111 face is the cleavage surface of diamond. Understanding its behavior is important from a technological point of view, the 111 plane is one of the growth surfaces for the chemical vapor deposition CVD of diamond, and from a theoretical point of view, in order to elucidate the principles of surface reconstructions in the other group IV semiconductors. Many potential applications of diamond arise from specific properties of its surfaces: while its negative electron affinity and unique surface conductivity make it attractive for electronic and optoelectronic devices, 1 its extreme surface chemical inertness makes it a good candidate for biomedical applications reducing the chances of rejection; moreover, boron-doped diamond electrodes have been used in water purification and in the detection of many substances, espe

Spin-polarized tunneling effects observed on the oxygen-terminated Fe3O4 (111) surface

Under oxidizing preparation conditions the magnetite (111) surface reconstructs to a highly ordered superlattice. This surface reconstruction represents an oxygen termination of the magnetite bulk. We employ spin-polarized (SP) scanning tunneling magnetization (STM) to study the spin-dependent tunneling between a magnetite (111) sample and an antiferromagnetic tip through a vacuum barrier. Atomic scale STM images show significant magnetic contrast corresponding to variations in the local surface states induced by oxygen vacancies. The local variations of the tunneling magnetoresistance around these vacancies correspond to 150%. By employing SP–STM measurements and first principles calculations we could conclude that an oxygen top layer considerably changes the SP properties of the magnetite surface.

Spatially resolved dynamics of the TiO 2( [formula omitted]) surface reconstruction

We use low-energy electron microscopy to image the reversible transformation of the TiO 2(1 1 0) surface between a high-temperature 1 × 1 structure and a low-temperature 1 × 2 structure. The reconstruction dynamics are novel: 1 × 2 bands nucleated during cooling at the steps of the starting 1 × 1 surface and then grew laterally from the steps. The transformation kinetics are dominated by mass flow from the surface to the bulk, a process that facilitates converting the high-density 1 × 1 phase to the lower-density 1 × 2 phase. We have also imaged how the 1 × 1 surface reconstructs to 1 × 2 phase after sufficient oxygen is removed from the crystal’s bulk during vacuum annealing. 1 × 2 bands also nucleated and grew laterally from the initial 1 × 1-surface’s steps. However, because this isothermal 1 × 1-to-1 × 2 transition occurs largely by mass redistribution on the surface, the steps of the initial 1 × 1 surface and final 1 × 2 surface are offset. We propose models of mass redistribution during the 1 × 1/1 × 2 phase transition to explain this effect. We conclude that the phase transition is first-order because it always occurred by the nucleation and growth of discrete phases. Finally, we show that quenching can roughen TiO 2’s surface by forming pits and that changing temperature causes step motion on 1 × 2 surfaces.

Spin-Dependent STM Tunnelling Study of the Patterned Magnetite (111) Surface

ABSTRACTUnder oxidizing preparation conditions the magnetite (111) surface reconstructs to a highly ordered superlattice. This surface reconstruction represents an oxygen-termination of the magnetite bulk. We employ spin-polarized (SP) STM to study the spin-dependent tunnelling between a magnetite (111) sample and an antiferromagnetic tip through a vacuum barrier. Atomic scale STM images show significant magnetic contrast corresponding to variations in the local surface states induced by oxygen vacancies. The local variations of the tunnelling magnetoresistance around these vacancies correspond to 150%. By employing SP-STM measurements and First principles calculations we could conclude that an oxygen top-layer considerably changes the SP properties of the magnetite surface. We explain the appearance of the superstructure in terms of electron-lattice instability due to the surface strain.

A study of a reverse engineering system based on vision sensor for free-form surfaces

Reverse engineering can quickly create a CAD model of a new product, in which, the sensor, sampling planning and surface reconstruction are three crucial elements. In this paper, a reverse engineering system involving a new vision sensor, an improved sampling planning module and a fine surface reconstruction module is developed. A characteristic of the proposed sensor is strong linearity between output and input, obtained by the structure optimization when a simple lens replaces the asperic lens. Back propagation (BP) neural network error compensation heightens accuracy. To increase efficiency of digitization, an improved sampling planning approach is proposed; it is based on surface curvature and tangent line slope of a measured point. In surface reconstruction, a new adaptive extracting approach based on curvature of surface reconstructs the non-uniform rational B-spline (NURBS) surface for the scattered data. The accompanying reverse engineering experiment proves the proposed system to be reliable and efficient.

Bridging the pressure gap in surface science at the atomic level: H/Cu(110).

The structural response of the Cu(110) surface to H2 gas pressures ranging from 10(-13) to 1 bar is studied using a novel high-pressure scanning tunneling microscope (HP-STM). We find that at H2 pressures larger than 2 mbar the Cu(110) surface reconstructs into the ( 1x2) "missing-row" structure. From a quantitative analysis of the pressure dependence of the surface reconstruction, we conclude that Cu(110) responds identically to hydrogen at ultrahigh vacuum conditions and at atmospheric pressures. From the HP-STM data, we extract refined values for the adsorption and desorption rate constants.

SURFACE FACETING ON Cu(115) INDUCED BY OXYGEN ADSORPTION

The clean Cu (115) surface shows the regular step arrangement of a vicinal surface in scanning tunneling microscopy. Upon oxygen adsorption the surface reconstructs completely. For temperatures around 500 K during or after oxygen adsorption, pronounced faceting into (104), (014) and (113) facets is observed. Details of the facet structures are analyzed. The (113) facet exhibits an additional reconstruction which can be described as a c(7×12) superstructure. The development of Cu–O–Cu atomic chains is suggested to be the driving force for the oxygen-induced surface reconstruction. This reconstruction is reversible; oxygen desorption re-establishes the initial (115) structure.

Hydroxy1 driven reconstruction of the polar NiO(111) surface

We have studied the reconstruction of the polar NiO(111) surface predicted recently by D. Wolf with low energy electron diffraction. Thin NiO films (10–20 Å) are used as substrates. As prepared, the films with a p(1 × 1) NiO(111) structure are covered with hydroxyl groups, which may be removed through a simple heat treatment. As the hydroxyl groups are desorbed, the surface reconstructs, exhibiting a diffuse p(2 × 2) structure. Readsorption of water onto the reconstructed surface lifts the reconstruction and again leads to the formation of the p(1 × 1) hydroxyl covered surface.

Arsenic coverage dependence of the angular distribution of secondary ions desorbed from the GaAs{001}(2×4) surface

In this paper, we present a study of structural changes which may occur as a function of surface composition within a single surface reconstruction. We examine in detail the GaAs{001}(2×4) surface by measuring the ion beam desorbed Ga+ ion angular distributions as a function of As coverage. The distributions of these ions, initiated by 3‐keV Ar+ ion bombardment at normal incidence, are measured as a function of azimuthal and polar angles of ejection from the molecular‐beam‐epitaxy‐grown (2×4) surface. The distributions display extreme anisotropy, indicative of a specific collision sequence which results from the covalent nature of the GaAs crystal. More subtle blocking and channeling effects are also found in the angular pattern which allow qualitative analysis of the (2×4) surface structure. These results agree with the currently accepted model of missing As dimers for this reconstruction, which appear as open channels parallel to the 〈011̄〉 direction on the surface. The As coverage of the (2×4) surface was modified by molecular beam deposition of either As or Ga. These results indicate structural variations within the range of As and Ga coverages which are considered to yield the (2×4) reconstruction. The excess Ga atoms appear to adsorb in the fourfold hollow sites on top of the As dimers and at least 0.25 monolayer of Ga can be deposited before the surface reconstructs to a (3×1) symmetry. It is proposed that this adsorbed Ga may destabilize the (2×4) surface by opening the dimers, and thus initiating the reconstruction. Excess As atoms appear to adsorb in the fourfold hollow sites of the open channels on the (2×4) surface. The As coverage of the (2×4) surface was also modified by ultrahigh vacuum annealing. Angle‐resolved secondary ion distributions and reflection high‐energy electron diffraction patterns indicate that the surface reconstructs. It is concluded that the use of molecular beam deposition to vary the surface As coverage may be more effective than the thermal desorption of As2 from a prepared surface.

Total-energy calculations for surface reconstructions in adsorbate systems

The reconstructive effect of adsorbed atomic monolayers of carbon and nitrogen on a nickel (001) surface is well known. The work presented here calculates the covalent part of the total energy of such a system, with a 0.5 monolayer coverage of adsorbate, to resolve alternative geometries with the same symmetry elements (p4g) and determine atomic positions. The result of the calculations is that the nickel surface reconstructs with energy savings of about 0.4 eV and 0.3 eV for the carbon and nitrogen adsorbates respectively. The effect of the reconstructive on the electronic characters of the system is examined using LCAO density of states calculations.

The structure of oxygen on W(001) by low energy ion scattering

The adsorption of oxygen on the W(001) surface has been studied by LEED, AES, and low energy alkali ion scattering in order to obtain a more detailed understanding of the short-range surface structure. Exposure at room temperature results in oxygen adsorption near bridge sites for all coverages below ∼ 0.6 ML as determined by the ion scattering incidence angle dependence. Above 0.6 ML, the oxygen adsorbs in four-fold hollow sites. When the oxygen-covered surface is annealed to 950 K, the surface reconstructs, leaving vacancies in the W lattice. This is confirmed by the observation of ions scattered from second layer W, through the vacancies, at relatively low angles of incidence. The oxygen atoms do not adsorb in the position vacated by the missing W atoms but instead adsorb atop the second W layer.

Direct observation of atomic structures in the surface reconstruction of Ir and Pt (001), (110), and (131) surfaces.

Atomic structures and atomic steps in the reconstruction of Ir and Pt (001), (110), and (131) surfaces, induced by nanosecond-pulsed-laser heating, have been observed in the field-ion microscope. The quasihexagonal arrangement of atoms in a (1\ifmmode\times\else\texttimes\fi{}5)-reconstructed Ir (001) plane is revealed for the first time by mapping the position of surface atoms in a reconstructed layer while it is gradually field evaporated. The Pt (001) surface reconstructs during annealing via a disordered phase. The final ordered phase has a structure similar to the (1\ifmmode\times\else\texttimes\fi{}5) structure of Ir(001). However, the closely packed atomic rows of the top layer are now rotated by a small angle from the [110] direction, and there is no quasihexagonal arrangement of atoms. Thus even though field-ion images of these surfaces are consistent with models proposed from studies using low-energy electron diffraction and other macroscopic techniques, there are a few significant differences in the detailed atomic arrangements. Pt and Ir (110) surfaces reconstruct from the (1\ifmmode\times\else\texttimes\fi{}1) to the (1\ifmmode\times\else\texttimes\fi{}2) structure by breaking of [110] atomic rows into fragments of two to several atoms and by lateral and cross-channel jumps of these fragments. The simultaneous jump of small rows of atoms is probably induced by surface soliton waves. The (131) surface of Pt and Ir also tends to reconstruct from the (1\ifmmode\times\else\texttimes\fi{}1) to the (1\ifmmode\times\else\texttimes\fi{}2) structure. The (1\ifmmode\times\else\texttimes\fi{}2) structure we have observed for Pt and Ir (110) and (131) surfaces is consistent with the ``simple'' missing-row model.