Physical Review B

Published by American Physical Society
Online ISSN: 1550-235X
Publications
Article
The variation of the potential of a quantum well is similar to that of a deep trap. In that respect a quantum well can capture and emit carriers in much the same way as a trap. The thermal emission energy from a quantum well is closely related to the appropriate band offset. With that in mind, we have carried out deep-level transient spectroscopy measurements on Schottky-barrier diodes containing one or more pseudomorphic InxGa1-xAs/Al0.2Ga0.8As (0<x≤0.18) quantum wells. The objective was to estimate the conduction-band offset, ΔEc, as a function of x and the resulting strain. From detailed balance between emission and capture, an Arrhenius-type expression was derived to analyze the transient emission data. It is seen that the percentage band offset ΔEc/ΔEg varies from 62% for x=0.07 to 70% at x=0.18. Furthermore, a linear interpolation of the data leads to ΔEc/ΔEg=58% at x=0, which is close to the widely accepted value. Our results support recent theoretical calculations from which a monotonic increase in ΔEc with strain in this heterostructure system is predicted.
 
Article
Neutron powder diffraction has been used to study the crystal structure and magnetic ordering of YBa2Cu3-xCoxO7+y with x=0.84 and y=0.32. Rietveld refinement of the crystal structure was carried out at 295 K (space group P4/mmm,a=3.888 A&#778,c=11.636 A&#778) and showed in particular that Co substitutes in the Cu(1) position of YBa2Cu3O7-y. Long-range, three-dimensional antiferromagnetism was observed up to a Néel temperature of about 405 K. All five detected magnetic peaks could be indexed on a wave vector ((1/2 1) / 2( 1/2) with h,k,l all odd, indicating a body-centered tetragonal magnetic unit cell with amag=a &surd;2, cmag=2c. At 9 K the proposed magnetic structure has moments parallel to c antiferromagnetic ordering within both Cu and Co layers. Adjacent moments in neighboring Cu layers are antiparallel whereas the small moments found in the Co layers are parallel to their adjacent moments in the neighboring Cu layers. The ordered moment was 0.87muB for the Cu layers and 0.16muB in the Co layers, respectively. With increased temperature the ordered moments in the Co layers decrease faster than those in the Cu layer. At room temperature the ordered moment was [0.65(7)]muB for the Cu layers with no detectable ordered moment in the Co layers.
 
Article
A comparison between bulk and films of Fe${(\mathrm{CO})}_{5}$ adsorbed on graphite shows that dynamical anomalies which occur at $\sim${}110, $\sim${}115, and $\sim${}180 K have in both forms similar effects on the M\"ossbauer parameters. The origin of these transitions is briefly discussed. In the 2.3-layer film, anomalous clustering of the film results in coexistence of solid bulk and a compressed monolayer; consequently, the film displays a novel behavior of the M\"ossbauer quadrupole splitting.
 
Intensity of the CAICISS spectral peak due to Bi as a function of time during Bi deposition on Si001 at 540 °C. The coverage of Bi was controlled at 0.36 0.02 ML. 
Intensity of the CAICISS spectral peak due to Bi during the deposition of Ge on Si001 at 450 °C. The coverage of Bi was kept constant at 0.24, 0.36, and 0.60 ML. 
RN Si /(N Ge N Si ) plotted against the amount of deposited Ge at various coverages of Bi a-d and Sb e-h. 
Article
The growth of Ge on Si(001) at elevated temperatures has been monitored in real time using coaxial impact collision ion scattering spectroscopy in the presence of various constant coverages of surfactant, Bi or Sb, with thermal evaporation of the surfactant being compensated automatically. As the coverage of surfactant increases, the intermixing of Ge and Si is suppressed, the crystalline quality of the resulting Ge film is improved, and the nucleation and growth of macroscopic Ge islands is suppressed.
 
Article
The Sb-Sb dimer structure of a 2×4 reconstructed Sb/GaAs(001) surface was investigated by core-level photoelectron collection mode of back reflection soft x-ray standing-wave technique. Sb atoms occupy the bridge site forming bonds with two underlying Ga atoms and form symmetric Sb-Sb dimers lining up in the [11¯0] direction. The first-layer Sb atomic plane is estimated to be 1.81±0.02 Å above the second-layer Ga lattice plane. The bond length of the Sb-Sb dimer on a GaAs(001) surface is estimated to be 2.95±0.06 Å, which is longer than the reported Sb-Sb bond length for Sb/GaAs(110) surface.
 
Article
We have performed linear and circular x-ray-dichroism measurements at the L2,3 edges of Ni in as-deposited and annealed layers on an Fe(001) substrate. The linear dichroism results suggest that the magnetic dipole term is small in these Ni layers. Upon annealing, the intermixing of Ni and Fe causes the Ni orbital moment to increase to twice the value for bulk fcc Ni. The possible origins of this increase are discussed. Fundamental importance is attributed to the higher localization of the Ni 3d states, due to the ``impurity'' nature intrinsic to Ni atoms diluted in Fe.
 
Article
Reconstructed surfaces on Sb/GaAs(001) have been investigated in situ by reflection high-energy electron diffraction (RHEED) and core-level photoelectron spectroscopy by heating a sample prepared by depositing Sb on an As-terminated GaAs(001) surface at room temperature (RT). Before Sb desorption, the halo RHEED pattern changes into 1×4, 1×3, and 2×4 patterns from RT to 560 °C, which is in contrast to Sb/GaAs(110) that shows only a 1×1 pattern. It is found that the GaAs surface with a 2×4 pattern is terminated by a monolayer of Sb, and that these superstructure changes are caused by As atom desorption followed by Sb atom substitution.
 
Article
Angle-resolved core-level photoelectron spectroscopy using a monochromatized synchrotron radiation source was used to study the structure of the Se-treated GaAs(001) surface. Depth profiling was achieved by varying the photoelectron angle with respect to the sample surface. The Se 3d spectrum was found to consist of two components. The higher-kinetic-energy component is assigned as the surface-sensitive component, which is opposite to the conventional assignment based on Ga coordination. The intensity ratios of the two Se components are calculated with a layer attenuation model, and the polar angle dependence shows that the two components cannot simply be assigned as ``surface'' and ``inner'' Se but are two different chemical states that exist at both the first and second Se layers, with only one chemical state existing at the third Se layer. Based on these results, we derived a Ga-vacancy zinc-blende structure where the first Ga layer has a vacancy occupancy of approximately 25%.
 
Article
Iron was substituted for copper in YBa2(Cu1-xFex)3O7- for 0x0.33. Superconducting transition temperatures and lattice parameters have been determined as a function of Fe content. The orthorhombic distortion a0=2(b-a)/(a+b) decreased to zero near x=0.03. Beyond x=0.03 the structure appears tetragonal. For compositions 0x0.15 the superconducting critical temperature Tc decreased smoothly from Tc=90 K to below 4.2 K. Transmission electron microscopy (TEM) revealed that the spacing between the twin boundaries decreased from 200 nm for x=0 to 20 nm for x=0.02 before the twins disappeared for x>=0.03. The sharp streaks in TEM diffraction patterns were interpreted as due to the twin boundary layers whose thickness varied from 1 nm for the pure to 2 nm for the Fe (x=0.02) containing specimen. A tweed structure in the TEM image and associated diffuse streaks along the 110 directions in electron diffraction pattern appeared for x>0.015. This observation suggests that the structure consists of fine (<5 nm) orthorhombic domains, each domain having the twinlike crystallographic relation with its neighbors. Rietveld refinement of powder neutron diffraction for x=0.10 and 0.167 determined that Fe substituted primarily on the Cu chain site, and for every two atoms of Fe substituted, approximately one extra oxygen is incorporated in the Cu-O plane.
 
Article
In order to understand the electronic structure of the misfit-layer compound (SnS)1.17NbS2 we carried out an ab initio band-structure calculation of the closely related commensurate compound (SnS)1.20NbS2. The band structure is compared with calculations for NbS2 and for hypothetical SnS with structure and interatomic distances as in (SnS)1.20NbS2. The calculations show that the electronic structure is approximately a superposition of the electronic structures of the two components NbS2 and SnS, with a small charge transfer from the SnS layer to the NbS2 layer. The interlayer bonding between SnS and NbS2 is dominated by covalent interactions. X-ray and ultraviolet photoelectron spectra were obtained for the valence bands. The observed spectra are in good agreement with the band-structure calculations.
 
Article
We have performed ab initio pseudopotential plane-wave calculations of both the atomic and electronic structure of the Ta (100) and (110) surfaces. The atomic relaxation of Ta (110) is calculated and our results for the structure of Ta (100) agree well with previous experimental reports. We find that the topmost interlayer separations contract relative to their bulk values by (12+/-1)% and (2+/-1)% for Ta (100) and (110), respectively. The changes in the second and deeper interlayer separations differ in sign. The calculated surface formation and relaxation energies are 1.92 eV/atom and -0.17 eV/atom for the relaxed Ta (100) surface and 1.18 eV/atom and -0.03 eV/atom for the relaxed (110) surface. We have also calculated the work functions for both surfaces and find values of 4.0+/-0.1 eV for (100) and 4.9+/-0.1 eV for (110), which are both in good agreement with experiment. Our study of the respective surface states and their variation as a function of the relaxation indicates that, in contradiction to a previous report, the surface states do not provide the driving force for the relaxations. We report our calculation of the [110] zone boundary phonon frequencies for bulk Ta. Lastly, this work demonstrates that the pseudopotential method can be applied accurately and efficiently even in the case of a column V transition metal such as Ta.
 
Article
In this paper we present polarization-, temperature-, and thickness-dependent x-ray-absorption measurements on extremely thin Dy overlayers. For Dy overlayers deposited at room temperature on Cu(110) and (magnetic) Ni(110) we observe strong linear dichroism. We deduce that the nature of the dichroic effects is predominantly electrostatic. When Dy is evaporated onto Ni(110) and kept at 50 K almost no dichroism is observed. Deposition of Dy on Cu(110) at 50 K, on the other hand, yields strong dichroism. We discuss these effects in greater detail and formulate a model for the overlayer system that seeks to explain the observed differences. Finally, a strong decrease of dichroism is observed as the Dy layer thickness increases, a phenomenon which is independent of the substrate. We propose a mechanism to explain this that involves counteracting contributions to the dichroism from both the overlayer-substrate and surface-vacuum interfaces. We illustrate this with an experiment involving a Ho probing layer.
 
Article
The structure of empty electronic levels in Xe on Ag(110) is probed by angle-resolved inverse photoemission. The observed dispersion indicates wetting behavior of a Xe film in (111) orientation on the Ag(110) surface. If the Ag(110) surface is precovered by a metallic potassium layer, the spectra indicate formation of a nonwetting Xe film. The data of the present experiment cast new light on the controversial interpretation of Xe optical spectra and promote the general understanding of wetting and nonwetting adsorption behavior.
 
Article
A seven-layer slab was used to model the ideal GaAs(110) surface. Two coverages, Θt=1/2 and 1 monolayer (ML) of Na atoms on the surface were studied. The total-energy calculations with the self-consistent pseudopotential method was used to determine the positions of Na atoms on the ideal GaAs(110) surface. We find that the charge transfer is from the Na to the Ga atoms. The shift of the Fermi level with respect to the valence-band maximum from coverage of 1/2 to 1 ML is 0.33 eV downward. This result agrees reasonably with the measured value of 0.2 eV for the n-type GaAs(110) surface.
 
Article
The unoccupied level structure of K and Na adsorbed on a Ag(110) surface has been probed by inverse photoemission. A peak is observed initially ~3 eV above EF, which shifts in energy towards the Fermi level as a function of increasing coverage. The dispersion of this feature is consistent with a pz character of the observed empty state. For CTHETA>0.3 new structures appear in the spectra indicating the formation of a metallic layer. Energy position, coverage dependence, and dispersion of the unoccupied level are in striking agreement with predictions made on the footing of previous ultraviolet photoemission spectroscopy, electron energy-loss spectroscopy, and theoretical results.
 
Article
Recent observations on various low-index metallic surfaces of an extraordinary thermal attenuation of diffracted peak intensities suggest that anharmonic effects are important at high temperatures, though such observations have also been interpreted in terms of disordering processes (premelting and roughening). In order to address this problem, we have carried out an extensive molecular-dynamics investigation of the evolution in temperature of the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom method potentials. We observe both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration; the latter are found in both cases to be larger in the plane of the surface than out of the plane. An analysis of our simulated low-energy electron-diffraction intensities supports the experimental interpretation of the measured ones in terms of enhanced mean-square displacements. Indeed, anharmonicities are found to appear at temperatures well below the onset of disordering, which occurs at 1200 and 1000 K for the (100) and (110) surfaces, respectively (via in-plane diffusion and adatom-vacancy formation). The onset of disordering proceeds differently on the two surfaces: while it involves essentially only the outermost layer on the (100) surface, both the first and second layer participate in the process on the (110) surface. We also calculate the phonon spectra at high-symmetry points of the surface Brillouin zone, which we find to be in good agreement with experimental values at room temperature. We predict, further, that the temperature-dependent frequency shifts are not significantly softened in comparison with bulk phonons, which suggests that surface phonons are affected by anharmonicity in a nontrivial way. Several of the observed properties can be understood in terms of the larger displacements that surface atoms can afford, because of their reduced coordination compared to bulk atoms.
 
Article
Temperature- and angle-dependent surface-extended x-ray-absorption fine-structure (SEXAFS) measurements on the system (2×3)N/Cu(110) are reported. Three inequivalent adsorption sites of the N atoms within the surface unit cell are identified. The successful separation of their contributions in the present SEXAFS analysis allows for a determination of N-Cu bond lengths not only within the surface plane but also to the second Cu layer. The temperature-dependent measurements between 60 and 300 K enable us to separate structural from dynamic effects in the SEXAFS amplitude, leading to a reliable determination of coordination numbers. A strong anisotropy is found in the adsorbate vibrations parallel versus perpendicular to the surface. The relative motion of the N atom relative to the first three Cu neighbor shells is characterized by means of the corresponding Einstein temperatures. These indicate that the N atom is primarily bonded only to four Cu atoms, and not to five as previously suggested. Anharmonic corrections are found necessary for a reliable distance determination of the N-Cu bonds to the second layer. The structural parameters are compared to the ones predicted by the pseudo-(100)-c(2×2)-N reconstruction.
 
Article
We have measured magnetic circular x-ray dichroism (MCXD) in a remanently magnetized Ni(110) single crystal, with the light in grazing incidence. Measuring the x-ray absorption as a function of the applied magnetic field, we could draw a hysteresis curve similar to one obtained with a magneto-optical Kerr effect on the same crystal. Spectra taken with linearly polarized light show clear variations as the angle of incidence of the light on the sample is changed. The possible origins for this angular-dependent absorption and its consequences for MCXD measurements taken in grazing incidence are discussed.
 
Article
The phonon spectrum of the Ag(110) surface along the direction is studied by He inelastic scattering with time-of-flight detection. The Rayleigh mode and two surface vibrational resonances are described up to the surface Brillouin-zone boundary. The dispersion curves of the resonances show the avoided crossing behavior predicted but never observed for the (110) surfaces of fcc metals. Surface lattice-dynamics calculations using a simple central nearest-neighbor force-constant model are performed and found to be in close agreement with the experiment.
 
Article
We report experimental evidence for strong 3{ital d}{r arrow}4{ital f} x-ray-absorption dichroism in rare-earth ions on a nonmagnetic substrate: Dy on Si(111)7{times}7. The results are discussed by evaluating the role of the crystal-field splitting of the ground state as an alternative to the Zeeman splitting underlying the magnetic x-ray dichroism theory.
 
Article
The valency changeover in Sm overlayers on Si(111)7×7 is probed with high accuracy using x-ray-absorption spectroscopy at the Sm M4,5 edges. The valency of Sm has been studied as a function of the layer thickness in the chemisorption regime, and compared with the results obtained for epitaxially grown samarium silicide layers.
 
Article
The in-plane structure of the Si(111):As-1×1 surface has been studied under ultrahigh vacuum using x-ray standing waves produced by a substrate silicon crystal in the grazing-angle diffraction geometry. The observed As fluorescence profiles are consistent with models indicating a bulklike surface with threefold-coordinated As atoms in the first layer. Evidence indicating that the As atoms occupy these high-symmetry sites with little disorder is obtained from investigation of the dependence of fluorescence profiles on the displacement and order parameters.
 
Article
In a molecular-beam-epitaxy (MBE) chamber, epitaxial growth of all the silicides existing in the low-temperature portion of the iron-silicon phase diagram [e.g., bcc Fe (+Si), FeSi, and the semiconducting silicide beta-FeSi2] was achieved on the (111) face of silicon by deposition of pure iron onto a heated silicon substrate. The epitaxial growth has been characterized by means of in situ reflection high-energy electron diffraction (RHEED), ultraviolet photoemission spectroscopy, and x-ray photoemission spectroscopy. A strained phase, s-FeSi2, has been clearly identified and shown to be metallic. This phase could be stabilized by the anisotropic elastic field induced by the epitaxy on the silicon (111) face which prevents a solid-state Jahn-Teller effect from distorting the cubic FeSi2 structure and opening the gap. Also during the growth, dynamical transitions between the different epitaxial phases have been observed at definite thicknesses (typically in the range 10-20 Å): (i) At the temperature of the silicon substrate T=350 °C, strained s-FeSi2-->FeSi (ii) at T=400 °C, strained s-FeSi2-->beta-FeSi2. In our work such dynamical transitions are experimentally observed in situ during the growth. This shows that the use of a MBE chamber and in situ RHEED are very powerful techniques for studying and controlling metallurgical transformations on nanometer scale in real time. We interpret these dynamical transitions as being due to the combination of two effects: a change in the silicon atomic flux coming from the silicon substrate to the surface and the relaxation of the strained phase.
 
Article
The influence of strong multiple interplanar forces is shown to give rise to a new class of surface vibrational resonances on metal surfaces as a result of higher-harmonic terms in the bulk-phonon dispersion relations. This effect is illustrated with the bcc Fe(111) surface where a surface resonance is observed at 21 meV by high-resolution electron-energy-loss spectroscopy, in excellent agreement with predictions by surface lattice-dynamics calculations.
 
Article
Surfaces of Sn growing on InSb\{111\}A,B have been studied by using the reflection high-energy electron-diffraction intensity oscillation technique. The surfaces proceed in the formation of a bilayered lattice in the whole range of film thickness. However, the geometry of the outermost surface layer is quite different in both systems: The growing surface on InSb(111)A smoothens with the same period as the lattice formation, whereas on InSb(111)B, below and above 6 ML of Sn, smooth surfaces emerge every period of monolayer and bilayer, respectively. The monolayer-period change in surface geometry is ascribed to Sb segregation on the growing surface.
 
Article
We have studied the surface reconstructions of the In/Ge(111) system by means of scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy. In atoms at the interface substitute the top-layer Ge atoms at all coverages and tend to relax downward, thus causing a compressive surface stress. To release the stress, some of the top-layer Ge atoms may be missing and some new lateral bonds may form between the second-layer Ge atoms. Depending on the concrete way of stress relief, which may vary with the In coverage, different surface reconstructions may form. Detailed atomic structural models for the striped and hexagonal structures of the system have been proposed for further studies. Comparing the information gathered from previous papers concerning the systems of group-III metals adsorbed on (111) surfaces of group-IV semiconductors, we suggest that the above mechanism might also be responsible for formation of the reconstructions of the III/IV(111) systems in general, at least when the coverage is around 0.5 ML.
 
Article
We have measured the ${\mathit{M}}_{4,5}$ edges of Er in submonolayer coverages of Er on Si(111), using linearly polarized x rays. The observed dichroism has been related to the surface crystal field and interpreted quantitatively in the hypothesis of an axial perturbation. The free ion $^{4}$${\mathit{I}}_{15/2}$ ground state is split over $\approxeq${}25 meV, the lowest level being of $\Vert${}\ifmmode\pm\else\textpm\fi{}15/2〉 symmetry. The results are compared to those of a previous experiment on Dy/Si(111).
 
Article
The relaxed geometries and electronic properties of the hydrogenated phases of the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surface are studied using first-principles molecular dynamics. A monohydride phase, with one H per dangling bond adsorbed on the bare surface, is found to be energetically favorable. Another phase where 43 hydrogens saturate the dangling bonds created by the removal of the adatoms from the clean surface is found to be nearly equivalent energetically. Experimental scanning tunneling microscopy and differential-reflectance characteristics of the hydrogenated surfaces agree well with the calculated features. \textcopyright{} 1996 The American Physical Society.
 
Article
In this paper, surface diffusion of Ge adatoms on the In-stabilized moderate temperature phase of Ge(111) was studied with a room-temperature scanning tunneling microscope and it has been found that, in addition to diffusion of individual adatoms neighboring to some defects, the majority of the moving adatoms forms strings or closed loops consisting of segments lying along directions. The mean lifetime of Ge adatoms on Ge(111) has been obtained, from which the activation energy barrier has been determined to be 0.83 +/- 0.02 eV. This experimental result of this quantity is in excellent agreement with its theoretical value for clean Ge(111) surfaces, thus showing not only that the energy barrier obtained here is a characteristic of clean Ge(111) surfaces, but also that there is no complicated collective motion involved in surface diffusion of Ge adatoms.
 
Article
The performance of different correlation functionals has been tested for alkali metals, Li to Cs, interacting with cluster models simulating different active sites of the Si(111) surface. In all cases, the ab initio Hartree-Fock density has been obtained and used as a starting point. The electronic correlation energy is then introduced as an a posteriori correction to the Hartree-Fock energy using different correlation functionals. By making use of the ionic nature of the interaction and of different dissociation limits we have been able to prove that all functionals tested introduce the right correlation energy, although to a different extent. Hence, correlation functionals appear as an effective and easy way to introduce electronic correlation in the ab initio Hartree-Fock description of the chemisorption bond in complex systems where conventional configuration interaction techniques cannot be used. However, the calculated energies may differ by some tens of eV. Therefore, these methods can be employed to get a qualitative idea of how important correlation effects are, but they have some limitations if accurate binding energies are to be obtained.
 
Article
Intrinsic time-of-flight hole mobilities (μ+) were obtained in naphthalene single crystals down to 4.2 K. Between 300 and 150 K the tensor component μaa+ increases with decreasing temperature T, obeying a μ∝T^n dependence, with n=-2.9. At low temperature the hole transport becomes nonlinear (sub-Ohmic) with the hole velocity tending to saturate with increasing electric field, E, at about 2×10^6 cm/s. The highest experimental μaa+ (obtained at the lowest E which allowed the observation of a distinct hole transit pulse) was 400 cm^2/V s at 10 K and 3 kV/cm. It will be shown that the low-temperature results can be understood in terms of a standard band-model description, whereas the continuation of the experimental temperature-dependence law (μ∝T^n) (for both holes and electrons) into the high-temperature regime remains a problem for future theoretical work. Electron transits were obtained down to 22.5 K [μaa- (22.5 K) = 24.5 cm^2/V s]. No field dependence of the electron mobility was detected.
 
Article
(151)Eu Mössbauer spectra in zero magnetic field of highly dilute (0.1 mol%) Eu(2+) ions in CaF(2) showed an almost temperature-independent asymmetrically split pattern, arising from the paramagnetic hyperfine interaction AS. I in a cubic crystal field with slow electron spin relaxation; in a small external magnetic field B of 0.2 T such that gμ(B)B>A an almost symmetrical pattern was observed. Both the spectra with and without external field are well described using the spin Hamiltonian and previous electron paramagnetic resonance data. A more concentrated (2 mol% Eu(2+)) sample exhibited a strongly broadened symmetrical resonance line due to an increased Eu-Eu spin relaxation rate; in an external magnetic field of 0.2 T the Mössbauer spectra exhibited further broadening and additional magnetic structures due to the reduced relaxation rate. When a large field of 6 T was applied such that gμ(B)B is much larger than the crystal field splitting, a fully resolved hyperfine pattern was observed at 2.5 K, with an effective field at the Eu nuclei of -33.7 T; at higher temperatures superimposed patterns originating from excited electronic states were observed in the spectra. The present results on the highly dilute CaF(2) : 0.1%Eu(2+) sample deliver a straightforward explanation for previous observations of a seemingly large dependence of the Eu(2+) isomer shift on europium concentration.
 
Article
The scintillation and ionization yields produced by 33.5 MeV/u 18O and 31.9 MeV/u 36Ar ions in neat liquid argon and in liquid argon with 10 ppm allene have been measured. Application of an external electric field of about 3 kV/cm leads to an initial enhancement of the scintillation by 17% in the neat liquid and as much as 22% in allene solutions. Scintillation and ionization yields generally are strongly dependent on field strength, they are similar for both ions, and they are usually very different from that found with alpha particles. Ionization yields are found to increase by more than an order of magnitude with the addition of 10 ppm allene. This increase occurs mainly because of the photoionization of allene by the liquid-argon scintillation. A considerable amount of quenching is observed in these systems and even with the huge increase in ionization allene is not very efficient in converting scintillation to ionic pairs. The potential energy resolution of allene-doped liquid-argon detectors is discussed.
 
Article
Measurements of the reversible magnetization in magnetic fields parallel and perpendicular to the c axis have been performed on magnetically aligned nearly-single-phase crystalline HgBa2Can-1CunOy (n = 1, 2, and 3) samples. The basic thermodynamic parameters describing the superconducting state, such as the penetration depth, the coherence length, the lower and upper critical fields, and their anisotropy, were determined. The anisotropy of HgBa2Can-1CunOy was found to increase as the number of the copper oxide layers in the unit-cell increases, although the magnitude of the anisotropy is rather small. Relatively smaller values of the thermodynamic critical field, in comparison with that of YBa2Cu3O7-δ, can cause the lower position of the irreversibility lines determined for the Hg-based superconductors.
 
Article
The microstructure of Bi2Sr2Can-1CunOy multilayers grown by molecular beam epitaxy on atomically flat SrTiO3 substrates has been studied by reflection high-energy electron diffraction (RHEED), atomic force microscopy, and x-ray-diffraction (XRD) techniques. The overall RHEED data, collected in situ at different Bi2Sr2CuOy/Bi2Sr2CaCu2Oy (2201/2212) multilayer growth stages, demonstrated a two-dimensional growth and rather a high quality of the interfaces. Following the evolution of RHEED patterns, some evidence of an increase in surface roughness after several multilayer periods, was detected. A one-dimensional x-ray-diffraction model was applied for a quantitative analysis of growth defects in the multilayers. The substitutional disorder in the lattice and stacking faults in the molecular layers were determined by an iterative comparison of simulated x-ray-diffraction spectra with the experimental XRD data. The observed changes in the c-axis lattice parameter of 2201 molecular layers were interpreted as being caused by ionic substitutions of Sr2+ by Ca2+ in the lattice and governed by the growth interdiffusion. The fitting procedure also revealed that two types of growth disorder were present in the layers: (1) stacking faults randomly distributed within the layers and (2) stacking faults localized at the interfaces. The two types of growth defect are expected to influence the superconducting properties differently and this has to be considered before the transport properties of superconducting multilayers are studied.
 
Article
Polarized reflection and ESR measurements have been carried out on single crystals of newly synthesized halogen-bridged one-dimensional (1D) metal complexes \{M(chxn)2Br\}Br2 (M=Pt, Pd, Ni; chxn=1R,2R-cyclohexanediamine), which have tight hydrogen bonds between ligands (chxn) and counter anions (Br-) and construct a two-dimensional (2D) hydrogen-bond network. Both the polarized reflection spectra and the temperature-dependent ESR signals indicate that the electronic state of M=Ni is essentially different from that of M=Pt (or Pd). From the analysis of these results, it can be concluded that the complex for M=Ni is in a monovalent state, where a Mott insulator is formed in a (-Ni3+-Br--) regular chain, in contrast to the mixed-valent state (-M2+-Br--M4+-Br--) for M=Pt and Pd. Thermally excited paramagnetic spins observed for the Pd complex can be explained by the soliton-kink model under the influence of the 2D hydrogen-bond network.
 
Article
Coherent neutron scattering is used to make a dynamical study of the disubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate) (pTS-D) in the monocrystalline monomer state. The aim of this study is to make a comparison with the dynamics of the same monocrystal in the polymer state [Phys. Rev. B 49, 11 602 (1994)]. The mechanism leading to the incommensurate phase of pTS-D monomer is also discussed. The experimental study shows an overdamped soft mode above Ti. Its evolutions as a function of temperature and its dispersion are described. A central peak related to the presence of defects is also observed. The soft mode presents roughly the same mean-field behavior for pTS-D monomer and pTS-D polymer. pTS-D mixed monomer-polymer crystals are then probably very good prototypes for the study of the influence of pure random field on a structural phase transition.
 
Article
Coherent neutron scattering and Raman scattering with IR sources were used to make a dynamical study of the structural transition of the disubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate) (pTS-D) in the monocrystalline polymer state. This compound may undergo a progressive polymerization in the crystalline state, which gives a quasiunique opportunity to obtain large size monocrystals of conjugated polymer. A second-order-like transition takes place at ~Tc=182 K; it is associated to motions of the polar side groups. We achieved a dynamical study of a soft mode which is clearly observed in the high-temperature phase (neutron) as in the low-temperature phase (Raman) and is characterized by overdamping on approaching Tc. In addition the growth of a central peak is observed above Tc. We show that this structural instability presents an intermediate character between order disorder and displacive, with a more pronounced displacive character than in the monomer crystal.
 
Article
The slowing-down process of Ti atoms in crystalline Ti and Si atoms in crystalline Si is studied using the molecular-dynamics method. Based on an experimental technique available for studies of the slowing-down process of atoms at ultralow velocities in bulk matter, the initial recoil energy was 261 eV for Ti and 677 eV for Si atoms. The values correspond to the energies imparted after the thermal neutron capture by primary γ rays. The effect of the crystalline structure on the slowing-down process is studied by calculating the energy distribution of the Doppler-shifted secondary γ rays emitted by the recoiling nuclei. The dependence of the distribution on the interatomic potential used in the simulations is also investigated.
 
Article
Ultraviolet and x-ray photoemission measurements have been performed on single-crystal (La1-xSrx)2CuO4 with x=0.0 and 0.04. Considerable differences are found in the photoemission spectra when compared with the results for sintered samples: a single-peak structure for the O 1s level and the absence of the peak at 9 eV of unknown origin reported for sintered samples, etc. Comparison is also made with band-structure calculations.
 
Parameters of the electron-electron interaction for LaM O 3 calculated assuming trivalent (M 3 ) and divalent (M 2 ) configurations for transition-metal ions as well treating only t 2g states as localized. 
Density of states for LaM O 3 perovskites with M Co-Cu obtained in LDAU 1 right panel and LDAU 2 left panel approaches. Dotted line corresponds to the LSDA. Position of the Fermi level is shown by vertical dashed line. The LDA U 1 result for LaNiO 3 corresponds to the antiferromagnetic configuration. 
Spectra of optical conductivity for LaM O 3 perovskites: LDAU applied only for t 2g and for all 3d states solid and dotted curves, respectively, LSDA dot-dashed, experiment by Arima et al. dashed. The LDAU 1 result for LaNiO 3 corresponds to the antiferromagnetic configuration. 
Article
Using the LDA+U method (where LDA is local-density approximation) we show that a separate treatment of t2g and eg electrons on transition-metal sites as localized and itinerant, respectively, gives an appropriate description for the band structure of LaMO3 perovskites (M=Ti–Cu) and systematically improves results of the local-spin-density approximation (LSDA) for the ground-state and single-electron excited-state properties. The analysis is based on comparison with experimental magnetic, optical, and photoemission data. Parameters of the effective Coulomb interaction estimated for t2g electrons and a role of eg screening are discussed. The present approach accounts well for the insulating natures of LaTiO3 , LaVO3 , and LaCoO3 , for which the LSDA predicts metallic states. Changes of the LSDA band structure for LaMnO3 and LaNiO3 are almost negligible due to the very efficient screening of on-site t2g interactions by eg electrons.
 
Article
We have studied the polarization dependence of the Cu 2p absorption spectra in a (Bi0.84Pb0.16)2Sr2CaCu2O8 single crystal. At normal incidence (E||xy plane) we found an intense peak at 932.0 eV, which becomes much weaker and shifts 500 meV toward lower energies at grazing incidence (E||z axis). Also, at grazing incidence a structured absorption step appears starting at 935.8 eV. The interpretation of these results is discussed.
 
Article
The ac susceptibilities of YBa2(Cu1-xFex)3O7-δ polycrystals for 0≤x<0.05 have been measured as a function of temperature and magnetic-field strength. Two different types of behavior of the irreversibility line, Tm(H), obeying the law 1-Tm/Tc∼Hq have been found. One corresponds to pure Y-Ba-Cu-O (x=0) and shows the commonly observed change of the power exponent q from 2/3 to 1 with increasing field. For the iron-doped samples an additional unusual crossover from q=1 to q=4/3 is observed practically within the same field and temperature interval as for the undoped one. The obtained results are described via the critical-state model taking into account the modification of the Josephson medium for the iron-doped case. In contrast to the silver-doped case [S.L. Shinde et al., Phys. Rev. B 41, 8838 (1990)], the pinning-force densities turn out to be smaller for iron-doped Y-Ba-Cu-O as compared to the undoped sample, probably due to iron-induced creation of a weak-link network inside grains.
 
Article
A simple cluster model calculation scheme which successfully describes the line shape and intensity ratio of recent spin resolved Fe 3s spectra of metallic Fe has been developed. From fitting the 3s spectra of bcc-bulk Fe, we found that the configuration interaction effect is important to the understanding of the 3s spectra. This model was applied to the Fe 3s spectra obtained from the ultrathin [0.7, 1.7, and 2 monolayer (ML)] Fe overlayers on Cu(100), and the 3s spectra of thin films were described well within our cluster model framework. Indeed the difference between the Fe 3s spectra of ultrathin films and bulk-bcc Fe was found to be consistent with the fact that the local magnetic moment of ultrathin films is smaller than that of the bcc bulk in our model. However, we did not find significant changes of the experimental 3s spectra between 0.7-, 1.7-, and 2-ML-thin films, which means that local magnetic moments of thin films do not change much below 2 ML.
 
Article
The dynamic magnetic correlations have been characterized in a large, orthorhombic single crystal of YBa2Cu3O6+x with x=0.6, having a Tc of 53 K. Inelastic-neutron-scattering measurements reveal a gap (zero magnetic cross section) of 5 meV in the spin-excitation spectrum at 10 K. The size of the gap fits well between those reported by Rossat-Mignod et al. for x=0.51 and 0.69. However, the rapid change in the gap size with a relatively small change in Tc is not understood.
 
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A model of the orientation phase transition in pure C60 is proposed. Eight discrete states of the fullerene molecule are considered, taking into account the interactions between the pentagons, hexagons, and double bonds of the neighboring molecules. The entropy gain of disorder is in good agreement with the differential-scanning-calorimetry results. The behavior of the order parameters is also in agreement with x-ray and neutron-diffraction measurements. However, the transition temperature obtained in our model with no adjustable parameters is larger than the experimental one.
 
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Positive muons injected into solid C70 form a vacuumlike muonium (Mu) state with a 25% probability. This state is attributed to muons inside the C70 cage (C70:Mu). In addition, three different types of muonated free radicals, distinguishable by their hyperfine coupling constants, are observed. These radicals are attributed to muonium addition on the outside of the C70 molecule at different sites. The hyperfine parameters of these radicals reflect the different degrees of delocalization of the unpaired electron. The temperature dependence of the depolarization rates yields information on the dynamics of the C70 molecules.
 
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Employing ab initio calculations we predict that the magnetic states of hydrogenated diamond-shaped zigzag graphene quantum dots (GQDs), each exhibiting unique electronic structure, can be selectively tuned with gate voltage, through Stark or hybridization electric-field modulation of the spatial distribution and energy of the spin-polarized molecular orbitals, leading to transitions between these states. Electrical read-out of the GQD magnetic state can be accomplished by exploiting the distinctive electrical properties of the various magnetic configurations.
 
Top-cited authors
Evgeny Tsymbal
  • University of Nebraska at Lincoln
Toni Schneider
  • University of Zurich
Gang Xiao
  • Brown University
Per Hyldgaard
  • Chalmers University of Technology
Erich P. Stoll
  • University of Zurich