Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Bulk-like structure of diamond {111}

A LEED (low-energy electron diffraction) intensity analysis of C {111} 1 × 1 (diamond) has given good agreement with a surface structure which has bulk positions with small relaxation of the first interlayer spacing for both insulating and semiconducting specimens. The truncated-bulk character of C{111} 1 × 1 gives support to the previously determined bulk-like structure of stabilized Si{111} 1 × 1, and casts doubt on the disordered 7 × 7 structure suggested for the stabilized 1 × 1 phase by interpretation of photoemission measurements. In both cases rather than doubt the LEED structure, one can doubt the interpretation of the photoemission measurements.     

LEED structure analysis of a Co{001} surface

A {001} surface of face-centered-cubic cobalt was cleaned to the point of elimination of all impurities except carbon and oxygen, which were reduced to minimum terminal amounts. A LEED structure analysis of this surface, using 12 intensity spectra at 3 angles of incidence, reveals that the atomic arrangement corresponds to truncation of the bulk structure but with about 4% contraction of the first interlayer spacing along 〈001〉 with respect to the bulk.     

On the atomic structure of {001}W

Results of a field ion microscope study of the clean surface structure of the {001} surface plane of tungsten are presented. The major conclusions are that {001}W is reconstructed over the temperature range 15–580 K, and that the reconstructed surface contains an alternating vertical component to the displacements of the W surface atoms. There are two magnitudes of verticaldisplacement, each arrayed with p(2 × 2) symmetry, and if the differences in the magnitude are neglected, the vertical displacements are arrayed in the well known (√2 × √2)R45° configuration. Details of this newly developed experimental approach for the study of surface reconstruction are reported, along with a number of control experiments which exclude the possibility that these results are artifacts due to the experimental technique. The discussion includes a comparison of the present results with those drawn from other experimental techniques, primarily low energy electron diffraction.     

A novel Sr_{3}Pb_{6}Ce_{2}Ti_{12}O_{36} ferroelectric thin film grown by pulsed laser ablation

Complex perovskite oxide ferroelectric thin films are of great technological interest because of their high dielectric constant and large tunability. In this paper, we report the structural and electrical properties of Sr \(_{3}\) Pb \(_{6}\) Ce \(_{2}\) Ti \(_{12}\) O \(_{36}\) (SPCTO) thin films grown by pulsed laser deposition. The role of oxygen pressure and substrate temperature on the microstructure, dielectric properties and leakage current mechanism of SPCTO thin films was investigated. Strong oxygen partial pressure dependence on the microcrystalline properties and leakage current conduction mechanism was observed. Both Raman spectra and C-V characteristics show a ferroelectric phase rather than paraelectric phase for the deposited thin films. Investigations on the leakage current showed that SPCTO thin films deposited at different oxygen pressure have different dominant conduction mechanism at various electric fields. The low field conduction mechanism is governed by Ohmic and space charge limited conduction mechanisms, whereas at high fields, the conduction process is dominated by Schottky emission mechanism. The dielectric constant as well as the tunability is found to increase with increase in the crystallite size.     

Characterization of the current-induced resistive spots in superconducting \hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7} strips

For over a decade, ultrathin superconducting films have been developed for the detection of single photons at optical or near infrared frequencies, with competitive performances in terms of quantum efficiency, speed, and low dark count rate. In order to avoid the requirement of helium refrigeration, we consider here the use of high temperature materials, known to achieve very fast responsiveness to laser irradiation. We excite thin filaments of the cuprate \(\hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7}\) by rectangular pulses of supercritical current so as to produce either a phase-slip centre (PSC) or a normal hot spot (HS), according to the temperature and the current amplitude selected. That procedure provides information about the maximum bias current to be used in a particle detector, about the return current back to the quiescent state after excitation, and about the rate of growth and decay of a HS. We also measure the time of PSC nucleation. A unique feature of that approach is to provide the rate of heat transfer between the film and its substrate at whatever temperature, in the superconducting state, in the practical conditions of operation.     

Determination of the c(2 × 2) structure of sulfur on Fe{001} by low-energy electron diffraction

In the early stages of reaction with sulfur, a clean Fe{001} surface develops a c(2 × 2) superstructure. A low-energy electron diffraction analysis of this structure leads to a model in which the S atoms lie in the four-fold symmetrical sites on the Fe{001} surface, the S-Fe interplanar spacing being $\text{1.09 ± 0.05}\text{A}\text{?}$ and corresponding to an effective radius of 1.06 Å for the chemisorbed S atoms. In contrast to Fe{001} 1 × 1-O, the first interlayer spacing of the substrate here is not significantly expanded.     

Scanning Josephson tunneling microscopy of single-crystal Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} with a conventional superconducting tip

We have performed both Josephson and quasiparticle tunneling in vacuum tunnel junctions formed between a conventional superconducting scanning tunneling microscope tip and overdoped Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} single crystals. A Josephson current is observed with a peak centered at a small finite voltage due to the thermal-fluctuation-dominated superconducting phase dynamics. Josephson measurements at different surface locations yield local values for the Josephson I_{C}R_{N} product. Corresponding energy gap measurements were also performed and a surprising inverse correlation was observed between the local I_{C}R_{N} product and the local energy gap.     

Superfluid density in a highly underdoped YBa_{2}Cu_{3}O_{6+y} superconductor

The superfluid density rho_{s}(T) identical with1/lambda;{2}(T) has been measured at 2.64 GHz in highly underdoped YBa_{2}Cu_{3}O_{6+y}, at 37 dopings with T_{c} between 3 and 17 K. Within limits set by the transition width DeltaT_{c} approximately 0.4 K, rho_{s}(T) shows no evidence of critical fluctuations as T-->T_{c}, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_{s} displays the behavior expected for a quantum phase transition in the (3+1)-dimensional XY universality class, with rho_{s0} proportional, variant(p-p_{c}), T_{c} proportional, variant(p-p_{c});{1/2}, and rho_{s}(T) proportional, variant(T_{c}-T);{1} as T-->T_{c}.     

Site connectivities in silver borophosphate glasses: new results from 11B{31P} and 31P{11B} rotational echo double resonance NMR spectroscopy

Local and medium range order in the glass system 50Ag2O-50[(B2O3)x-(P2O5)(1-x)] (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) have been investigated by high-resolution solid state nuclear magnetic resonance (NMR) techniques. The detailed local site distribution has been derived from deconvolution analysis of the 11B and 31P magic-angle spinning (MAS) NMR signals. Quantitative information regarding the extent of boron-oxygen-phosphorus connectivity has been obtained on the basis of 11B[31P} and 31P{11B} rotational echo double resonance experiments. Incorporation of borate into silver metaphosphate glasses produces four-coordinate BO4/2- sites, which crosslink the metaphosphate chains, resulting in a significant increase in the glass transition temperature. Furthermore, the presence of borate favors the disproportionation of P(2) chain-like units into P(1) and P(3) sites, an effect not observed in binary alkali phosphate glasses. Finally, borate incorporation beyond x=0.3 results in the formation of neutral BO3/2 units, indicating some net charge transfer from the borate to the phosphate network former species. This latter result corresponds to the general metal ion scavenging effect observed for phosphate species in other mixed network former glass systems. In the present system, the effect is relatively moderate, however, suggesting that anionic BO4/2- groups are stabilized by the interaction with the phosphate groups.     

Preliminary studies of the Raman spectra of \hbox {Ag}_{2}\hbox {Te}\hbox { and }\hbox {Ag}_{5}\hbox {Te}_{3}

The theoretical calculations indicated that the monoclinic low-temperature phase of silver telluride $(\upbeta \hbox {-Ag}_{2}\hbox {Te})$ is a new binary topological insulator with highly anisotropic single Dirac cone surface. We obtained $\upbeta \hbox {-Ag}_{2}\hbox {Te}$ crystal ingots containing few grains by the Bridgman method. We also deposited thin films of tellurium, $\hbox {Ag}_{5}\hbox {Te}_{3}\hbox { and }(\hbox {Te+Ag}_{5}\hbox {Te}_{3})$ by thermal evaporation method. The Raman spectra of $\upbeta \hbox {-Ag}_{2}\hbox {Te}$ , tellurium and $\hbox {Ag}_{5}\hbox {Te}_{3}$ were measured at three excitation wave lengths: 633, 515 and 488 nm. The Raman active modes of $\upbeta \hbox {-Ag}_{2}\hbox {Te}$ , tellurium and $\hbox {Ag}_{5}\hbox {Te}_{3}$ are situated at frequencies below 300  $\hbox {cm}^{-1}$ while vibrations of other phases appear at higher frequencies.     

Direct observation of the atomic structure of W {100} surfaces

Fully resolved atomic images of the W {100} plane are obtained with the field ion microscope at 21°K. The image structure is consistent with the (1 × 1) structure. If shifts of atoms in the 〈110〉 directions parallel to the plane have occurred as derived from LEED investigations in the temperature range of 100–370°K, then the shifts at 21°K are too small to be observed in the FIM.     

Epitaxial growth and surface structure of NiSi2{0 0 1} on Si{0 0 1}

LEED (low-energy electron diffraction) intensity measurements were done to monitor the formation of epitaxial NiSi₂ on the {0 0 1} surface of Si. The resulting silicide surface, although possibly reconstructed, was subjected to LEED intensity analysis as NiSi₂{0 0 1}1×1. Several structural models involving bulk-like terminations, and variants thereof, failed the LEED test. However, one model, with Si adatoms in the four-fold symmetrical sites of the Si bulk-like termination, provides reasonable agreement with experiment and is therefore tentatively proposed as containing the main features of the real structure. The model may explain the failure of double epitaxy, i.e., the failure to grow epitaxial Si films on the silicide surface.     

Doping-induced change in the interlayer transport mechanism of Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} near the superconducting transition temperature

We perform a detailed study of temperature, bias, and doping dependence of interlayer transport in the layered high temperature superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+delta}. We observe that the shape of interlayer characteristics in underdoped crystals exhibits a remarkable crossover at the superconducting transition temperature: from thermal activation-type above T_{c} to almost T-independent quantum tunneling-type below T_{c}. Our data provide insight into the nature of interlayer transport and indicate that its mechanism changes with doping: from the conventional single quasiparticle tunneling in overdoped to a progressively increasing Cooper pair contribution in underdoped crystals.