Superparamagnetism in Heterogeneous AgFe Thin Films – A Low Energy μSR Study

This paper describes a study of superparamagnetism in iron nanoclusters using low energy μSR, including for the first time zero field measurements, which yield an intrinsic nanocluster superparamagnetic relaxation rate of ν₀=67±15 MHz and an energy barrier of ΔE=37±4 K. TEM micrographs of the embedded clusters show there to be a range of shapes and also sizes, which is bourne out by SQUID magnetometry measurements. This revised version was published online in September 2006 with corrections to the Cover Date.     

Defect-level analysis of semiconductors by a new differential evaluation ofn(1/T)-characteristics

A new method for the defect-level analysis of extrinsic semiconductors is described. Provided that the defect-level concentration is not too large and the temperature is not too low, the Fermi levelE _F is shifted with increasing temperature from a position near the conduction (or valence) band towards the middle of the forbidden gap monotonously. Thus majority carriers are emitted into the conduction (or valence) band from the defect levels successively. If for a small increment of the temperature the Fermi levelE _F is shifted by ΔE _F and the concentration of free majority carriers is increased by Δn, then the ratio Δn/ΔE _F is a measure of the defect-level concentration within ΔE _F . Furthermore we discuss how this analysis is influenced by additional defect levels outside the range over which the Fermi energy can be shifted by variation of the temperature.     

Eigenvalues of Laplacian with Constant Magnetic Field on Non-Compact Hyperbolic Surfaces with Finite Area

We consider a magnetic Laplacian −Δ _A = (id + A)^* (id + A) on a non-compact hyperbolic surface M with finite area. A is a real one-form and the magnetic field dA is constant in each cusp. When the harmonic component of A satisfies some quantified condition, the spectrum of −Δ _A is discrete. In this case, we prove that the counting function of the eigenvalues of −Δ _A satisfies the classical Weyl formula, even when dA=0.     

Transformation of auger electron spectra of ytterbium nanofilms under the action of adsorbed carbon monoxide and oxygen molecules

The transformation of the Auger electron spectra of ytterbium nanofilms as a result of chemisorption of CO and O₂ molecules on their surface has been studied. It has been shown that the adsorption of these molecules is accompanied by a radical transformation of the electronic structure of nanofilms, during which the 5d level of ytterbium drops below the Fermi level. As a consequence, one electron can transfer from the 4f level to the 5d level. In turn, this provides the conditions for a giant resonance 4d → 4f and a subsequent Coster-Kronig supertransition 4d ⁹4f ¹⁴ → 4d ¹⁰4f ¹² + Auger electron, which is accompanied by emission of one 4f electron to vacuum. The results obtained have demonstrated that molecules chemisorbed on the surface of nanofilms can cause qualitative changes in the properties of the surface and in the bulk of these films. It is obvious that this offers a means for designing nanoobjects with controlled properties.     

Size-controlled Pd nanocluster grown by plasma gas-condensation method

Syntheses of palladium (Pd) nanoclusters by sputtering gas-condensation technique are reported in this work. The Pd nanocluster size distribution and number of nanoclusters are found to depend on several factors which include the inert gas flow rate (f), sputtering discharge power, and growth region length. The discharge power and the length of the growth region are optimized to produce high nanocluster yield. It is found that low f produces small number of large nanoclusters which can be attributed to the formation of nanoclusters via the two-body collision mechanism. For high f, nanocluster size decreases and the number of nanocluster increases with f. This phenomenon can be assigned to nanocluster formation through three-body collision mechanism.     

On the use of Fourier-transformed Compton profile for the determination of band occupancies in certain solids

The Pattison-Williams method of using Fourier-transformed Compton profiles for obtaining Fermi momenta in simple metals, is extended to set up a scheme for inferringd orf band occupancies in transition metals, rare earths etc. CP measurements are reported for Ni,α-Mn andβ-Mn and then the new scheme is applied to obtain 3d occupancies in these systems. Some comments on the use of this method for other systems are also made.     

Singularity in the LEIS spectrum of metal nanoclusters

The asymmetry of lines has been observed in the spectra of the inelastic scattering of low-energy ions interacting with nanoclusters of d metals. It is attributed to the mechanism of exciting e-h pairs with low energies and high momenta (infrared catastrophe). Analysis of the asymmetry of x-ray photoelectron spectra and spectra of low-energy scattered ions shows that the Anderson singularity indices α associated with the electron states on the surface and in bulk have different behaviors as functions of the nanocluster size. The results cannot be explained in the framework of the available concepts of the redistribution of these contributions with decreasing of the nanocluster size.     

The position of the 4f level in Ce metal

The available spectroscopic data for Ce and its compounds are evaluated and it is concluded that in the groundstate the Ce 4f level is situated probably notmore than 1 eV below the Fermi energy. This does not exclude that with photoelectron spectroscopy an excited 4f hole state can be observed 2 eV or more below the Fermi energyE _F .     

Investigation of the characteristic energy losses of platinum (111)

With the help of a combined LEED- and Auger-investigation, the surface of a platinum (111) crystal was cleaned first. Then, the spectrum of the characteristic energy losses for both contaminated and cleaned surfaces is studied. On the cleaned surface the following losses were found: ΔE ₁=7.4 eV, ΔE ₂=13.5 eV, ΔE ₃=24.8 eV, ΔE ₄=31.8 eV, ΔE ₅=45.1 eV, ΔE ₆=54.1 eV, ΔE ₇=71.2 eV. The present results are compared with the measurement of other investigators. In particular, in good agreement with optical measurements we identify ΔE ₁ and ΔE ₂ as interband transitions, and ΔE ₃ and ΔE ₄ as surface and volume plasma loss, respectively.     

Ab initio study of structural, electronic, optical, and vibrational properties of ZnxSy (x?+?y?=?2 to 5) nanoclusters

An ab initio study of the stability, structural, electronic. and optical properties has been performed for 46 zinc sulfide nanoclusters Zn _x S _y (x + y = n = 2 to 5). Five out of them are seen to be unstable as their vibrational frequencies are found to be imaginary. A B3LYP-DFT/6-311G(3df) method is employed to optimize the geometries and a TDDFT method is used for the study of the optical properties. The binding energies (BE), HOMO-LUMO gaps and the bond lengths have been obtained for all the clusters. For the ZnS₂, ZnS₃, and ZnS₄ nanoclusters, our stable structures are seen to be different from those obtained earlier by using the effective core potentials. We have also considered the zero point energy (ZPE) corrections ignored by the earlier workers. For a fixed value of n, we designate the most stable structure the one, which has maximum final binding energy per atom. The adiabatic and vertical ionization potentials (IP) and electron affinities (EA), charges on the atoms, dipole moments, optical properties, vibrational frequencies, infrared intensities, relative infrared intensities, and Raman scattering activities have been investigated for the most stable structures. The nanoclusters containing large number of S atoms for each n is found to be most stable. The HOMO-LUMO gap decreases from n = 2–3 and then increases above n = 3. The IP and EA both fluctuate with the cluster size n. The optical absorption is quite weak in visible region but strong in the ultraviolet region in most of the nanoclusters except a few. The optical absorption spectrum or electron energy loss spectrum (EELS) is unique for every nanocluster and may be used to characterize a specific nanocluster. The growth of most stable nanoclusters may be possible in the experiments.     

Electron impact single ionization of copper

Electron impact single ionization cross sections of copper have been calculated in the binary encounter approximation using accurate expression for σ _ΔE as given by Vriens and Hartree-Fock momentum distribution for the target electron. The BEA calculation based on the usual procedure does not show satisfactory agreement with experiment in this case but a striking modification is found to be successful in explaining the experimental observations. The discrepancy is linked with the ionization of the 3d ¹⁰ electrons and probably effective single ionization does not take place from 3d shell of copper leading to smaller values of experimental cross sections.     

Low-energy electron spectrum in copper oxides in the multiband p-d model

An exact diagonalization of the Hamiltonian in the p-d model of a CuO₆ cluster was used to obtain dependences on the model parameters of the lowest-energy two-hole terms: the energy difference between the 2p orbitals of planar and apical oxygen Δ(apex)=ε(2p)−ε[2p(apex)], the crystal field parameter , and the ratio of the distances between the copper atom and the apical and planar oxygen atoms d(apex)/d(pl). In the limit of large d(apex)/d(pl) and Δ_d, our model is equivalent to the three-band p-d model and, in this case, large singlet-triplet splitting Δε⩾1 eV is also observed. As the parameters decrease, a singlet-triplet crossover is observed. Two mechanisms are identified for stabilization of the triplet term ³ B _1g (0) as the ground state. It is shown that for realistic values of the parameters, reduction of the p-d model to the three-band model is limited by the low energies of the current excitations because of the presence of the lower excited ³ B _1g and ¹ A _1g cluster states. Intercluster hopping causes strong mixing of singlet and triplet states far from the G point. The results of the calculations are compared with data obtained by angle-resolved photoelectron emission in Sr₂CuO₂Cl₂. Fiz. Tverd. Tela (St. Petersburg) 40, 184–190 (February 1998)     

d-symmetry superconductivity due to valence bond correlations

It is shown that d-symmetry superconductivity due to valence bond correlations is possible. Valence bond correlations are compatible with antiferromagnetic spin order. In order to explictly construct a homogeneous state with the valence bond structure in the two-dimensional Hubbard model for an arbitrary doping, we have used the variational method based on unitary local transformation. Attraction between holes in the d-channel is due to modulation of hopping by the site population in course of the valence bond formation, and corresponding parameters have been calculated variationally. An important factor for the gap width is the increase in the density of states on the Fermi level due to antiferromagnetic splitting of the band. The gap width and its ratio to the T _c are 2Δ≃0.1t and 2Δ/kT _c≃4.5−4 for U/t≃8. The correspondence between the theoretical phase diagram and experimental data is discussed. The dependence of T _c on the doping δ=|n−1| and the Fermi surface shape are highly sensitive to the weak interaction t′ leading to diagonal hoppings. In the case of t′>0 and p-doping, the peak on the curve of T _c(δ) occurs at the doping δ _opt, when the energy of the flattest part of the lower Hubbard subband crosses the Fermi level at k∼(π,0). In underdoped samples with δ<δ _opt, the anisotropic pseudogap in the normal state corresponds to the energy difference |E(π,0)−μ| between this part of the spectrum and the Fermi level. Zh. éksp. Teor. Fiz. 114, 985–1005 (September 1998)     

Common vacuum conservation amplitude in the theory of the radiation of mirrors in two-dimensional space-time and of charges in four-dimensional space-time

The changes in the action (and thus the vacuum conservation amplitudes) in the proper-time representation are found for an accelerated mirror interacting with scalar and spinor vacuum fields in 1+1 space. They are shown to coincide to within a factor of e ² with changes in the action of electric and scalar charges accelerated in 3+1 space. This coincidence is attributed to the fact that the Bose and Fermi pairs emitted by a mirror have the same spins 1 and 0 as do the photons and scalar quanta emitted by charges. It is shown that the propagation of virtual pairs in 1+1 space can be described by the causal Green’s function Δ_f(z,μ) of the wave equation for 3+1 space. This is because the pairs can have any positive mass and their propagation function is represented by an integral of the causal propagation function of a massive particle in 1+1 space over mass which coincides with Δ_f(z,μ). In this integral the lower limit μ is chosen small, but nonzero, to eliminate the infrared divergence. It is shown that the real and imaginary parts of the change in the action are related by dispersion relations, in which a mass parameter serves as the dispersion variable. They are a consequence of the same relations for Δ_f(z, μ). Therefore, the emergence of a real part in the change in the action is a direct consequence of causality, according to which Re Δ_f(z,μ)≠0 only for timelike and lightlike intervals. Zh. éksp. Teor. Fiz. 116, 1523–1538 (November 1999)     

Chemical states of Bi-doped GeTe (Bi: 6 at.%) thin film in structural phase transition investigated by synchrotron X-ray photoelectron spectroscopy

The chemical states of Bi (6 at.%)-doped GeTe thin film (GBT) in structural phase transition were investigated by high-resolution X-ray photoelectron spectroscopy (HRXPS). Clean amorphous GBT was phase changed to the NaCl-type crystalline structure by in-situ annealing under ultrahigh vacuum, during which transition, the binding energy and shape of the Te 4d core-level showed no changes, the Ge 3d core-level showed a spin-orbit split-enhanced feature with negligible chemical shift, and the Bi 4f core-level became narrower and shifted towards the higher binding energy side by 0.25 eV. We suggest that as the film phase changed to the meta-stable crystalline structure, the Ge and Bi atoms moved to more constrained sites in the electron configuration.     

Electron yield per ion charge-state correction for an ion collector with unsuppressed secondary electron emission

The electron yield per ion charge-state γ/q was measured for emission of electrons from clean polycrystalline gold induced due to impact of Ta ^q+ (11≤q≤41) ions with kinetic energy per chargeE _i/q from 15 keV/q to 150 keV/q. The dependence of γ on angle of incidence was analyzed with use of relation γ(ϑ)=γ₀ cos^−f ϑ. The fitting of experimental data gives a range of γ₀/q from 1 to 1.75 for Ta¹³⁺ and from 1.5 to 1.73 for Ta³⁹⁺. The dependence of γ₀/q onq andE _i is discussed with respect to measurement of ion currents emitted from laser-produced plasmas with an ion collector with unsuppressed secondary electron emission. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy, and by grant A1010819 from the Grant Agency of the Academy of Sciences of the Czech Republic.     

An approximate expression for the galvanomagnetic tensor

Using the iterative solution to the Boltzmann equation for electrons in d.c. electric and magnetic fields, an expression for the resistivity tensor can be obtained in the form of an infinite series. This series can be approximated by retaining only the first two terms. In the cases where relaxation times exist — in the sense that the collision term in the Boltzmann equation can be written asg(k)/τ(k), whereτ(k) is the relaxation time, andf (k) = f _E(ɛ k) + [∂f _E(εk)/∂ε]·g(k) the distribution function for electrons with wavevectork — this approximation is exact. For polyvalent metals in the one-OPW approximation, the complete galvanomagnetic tensor can be obtained using this approximation and the result differs from that obtained by using a time of relaxation given by an expression suggested byZiman. A calculation for a simple model Fermi surface, with screened Coulomb scattering, is carried out and the results compared with those of the relaxation time approximation.     

Dispersion of quasi-particles in high Tc cuprates

The interplay between superconductivity (SC) and antiferromagnetism (AFM) is studied in strongly correlated systems of high T _c Cuprate superconductors. It is assumed that superconductivity arises due to BCS pairing mechanism in presence of AFM in Cu lattices of Cu-O planes. The total Hamiltonian of the system is mean field one and has been solved exactly by writing the equations of motion for the single particle Green’s functions. Equations for the appropriate single particle co-relation functions are derived and the order parameters corresponding to SC and AFM are determined. It is assumed that the Fermi energy ∈ _F = 0 and the renormalized localized f energy level coincide with the Fermi level. All the quantities in the final equation for h and Δ are made dimensionless by dividing by 2t, where t is the hopping integral. The temperature dependent values of staggered magnetic field (h) and SC gap (Δ) were determined by solving self-consistent equations for h and Δ. The quasiparticle energy bands are function of AFM gap (h), SC gap (Δ) and hybridization (V). Then the dispersion of quasi-particles are studied at different temperatures by considering temperature dependent values of h and Δ and varying other different model parameters.      

Magnetic field dependence ofT c and temperature dependence ofH c2 in layered superconductors with open normal state Fermi surface

A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |H _c2(T)|,T ∼ 0 andT →T _c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|H_c2(0)|/[-Tδ|H _c2 T|δT]_T{s0} for the case of a FS open ink _z-direction with thek _z-bandwidth, 4t ₃, very small compared to the Fermi energy,E _F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofT _c and the temperature dependence of H _c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa₂Cu₃O₇ used elsewhere, we find reasonable values for the upper critical fieldH _c2(0), the slope (dH _c2/dT)_{T c0}, anisotropic coherence lengths ζ_i(T=0),i=x, y, z, and (dT _c/d|H|)_{|H| → 0}.     

Simulation of desorption of gold nanoclusters deposited on the (111) surface of Al and Au under bombardment with Au<Subscript>1</Subscript> ions and Au<Subscript>400</Subscript> clusters using the classical molecular dynamics method

The mechanisms of desorption of gold nanoclusters deposited on à substrate under low-energy bombardment with ions and clusters (in the mode of elastic stopping predominance) are analyzed. The classical molecular dynamics method is employed for computer simulation of both “direct” and “indirect” impact of bombarding particles (that is, when a projectile particle either directly interacts with the deposited cluster or penetrates into the substrate without hitting the cluster). Au₁ ions and Au₄₀₀ clusters with an energy of 38 keV and 0.18 keV/atom, respectively, are used as projectile particles. The spherical Au₆₀₅₁ gold nanocluster deposited on a substrate of Al(111) or Au(111) is applied as a target. It is shown that indirect impact does not lead to desorption of the nanocluster from the Al substrate in all considered cases; however, it can initiate desorption from the Au substrate. This phenomenon is quite efficient when the heat spike appearing upon penetration of a projectile particle involves the region of contact between the substrate and the nanocluster deposited on it. As this takes place, an intense flow of the sputtered substrate material transfers a sufficient-for-ejection momentum to the deposited nanocluster.