Table of contents

It all began in 1979 when Bruce Doak decided to leave MIT after a year of graduate school to come to Göttingen to do something new. Within a year he succeeded in putting together a novel helium atom surface scattering apparatus, with which the first surface phonon dispersion curves were measured on the LiF surface out to the zone boundary [1]. To help us understand these results we invited Giorgio Benedek to Göttingen in June 1980. Giorgio then was a regular guest in the lattice dynamics theory group of Heinz Bilz, a director at the Max Planck Institut für Festköroperforschung in Stuttgart. Heinz Bilz at that time was developing models for phonons in metals in which the electron degrees of freedom were modeled by assigning multipole deformabilities to the ion cores [2]. This explains his excitement, when in 1983 he heard through Giorgio Benedek that another PhD student, Ulrich Harten [3], had succeeded in another apparatus (HUGO I) in our Institut to measure the surface phonon dispersion curves on Ag(111) [4]. Both Benedek and Bilz were especially fascinated by the discovery of a second dispersion curve at frequencies above the ubiquitous Rayligh mode. This prompted Bilz to organize on short notice an informal gathering in his Institut on `Oberflächenstatistik and dynamik'. My opening lecture on the new experiments was followed by six half hour theoretical lectures including talks by Fritz de Wette and by Giorgio Benedek, the pioneers in realistic calculations of surface dispersion curves on alkali halide surfaces. This was the birthday of the Surphon Series. The official conference names, organizers, venues, dates and numbers of participants of all the Surphon meetings held since are listed below:

• Statics and Dynamics of Surfaces, H Bilz (Max-Planck-Insitut für Festkörperforschung, Stuttgart, 27 September 1983) 7 speakers

• Statics and Dynamics of Surfaces, J P Toennies (Max-Planck-Insitut für Strömungsforschung, Göttingen, 15 June 1984) 11 speakers, 31 participants

• Statics and Dynamics of Surfaces, H Bilz and W Kress (Max-Planck-Insitut für Festkörperforschung, Stuttgart, 27--28 June 1985) 12 speakers

• Workshop on Surface Phonons , J P Toennies and W Kress (Ringberg Schloss, Rottach-Egern, 22--25 June 1987) 24 speakers, 38 participants

• Workshop on Surface Phonons, J P Toennies and W Kress (Ringberg Schloss, Rottach-Egern, 24--28 June 1990) 25 speakers, 38 participants

• Workshop on Surface Phonons, J P Toennies and W Kress (Ringberg Schloss, Rottach-Egern, 24--28 May 1992) 26 speakers, 38 participants

• Workshop on Dynamical Phenomena at Crystal Surfaces, D L Mills ( Countryside Inn, Costa Mesa, CA, USA, 27 June--1 July 1994) 33 speakers, 45 participants

• Workshop on Surface Dynamics Adsorbate Vibrations and Diffusion, J P Toennies and W Kress (Ringberg Schloss, Rottach-Egern, 18--21 June 1997) 30 speakers, 40 participants

• Workshop on Surface Dynamics, V Celli, A Kara, T Raman and J Skofronik (University of Virginia, Charlottesville, USA, 2--6 June 1999) 24 speakers, 35 participants

• Workshop on Surface Dynamics Phonons, Adsorbate Vibrations and Diffusion, D Farias and S Miret-Artes (Eroforum Hotel, l Escorial, Spain, 13--17 June 2001) 20 speakers, 36 participants

• Workshop on Surface Dynamics, Phonons, Adsorbate Vibrations and Diffusion, M Bertino (Meramec Park, Sullivan, MO, USA, 2--5 October 2003) 25 speakers, 33 participants

Speaking for the late Heinz Bilz and others attending the first meeting a little over 20 years ago it is indeed gratifying to witness the strong continued interest in surface dynamics and the close personal contacts among the congenial group of second generation surphon enthusiasts. We wish them lots of satisfying scientific success and many more exciting surphon meetings.

J Peter Toennies

References

[1] Brusdeylins G, Doak R B and Toennies J P 1980 Rev. Phys. Lett.44 1417

Brusdeylins G, Doak R B and Toennies J P 1981 Rev. Phys. Lett.46 437

[2] See for example:

Bilz H, Güntherodt G, Kleppman W and Kress W 1979 Phys. Rev. Lett.43 1998

[3] Ulrich Harten was another enterprising student who forsook a safely initiated PhD project at another German university to join us in this new venture

[4] Doak R B, Harten U and Toennies J P Phys. Rev. Lett.51 578

The following papers represent about one-half (twelve out of twenty-five) of the presentations at the Surphon 11 Workshop held at Meramec, Missouri in October 2003. Papers of the preceding (2001) workshop were published as a special issue of Journal of Physics: Condensed Matter (2002 volume 14, issue 24). The announced scope of these workshops, Surface Dynamics, Phonons, Adsorbate Vibrations, and Diffusion, is so broad that one may question whether there is an underlying unity to the meetings. The answer is yes, and the discussions and comments on the contributions, while not reported here, met the goal of fostering constructive interactions. In the following, I attempt to illustrate this.

One general feature of the work is common to research in most of modern condensed matter physics and surface science in particular. The systems under discussion are tailored materials or otherwise controlled at the (sub)nanometer length scale. There is an immense variety for the choice of substrate, face exposed by the substrate, and the adsorbate. Some effort is in effect a service, helping to characterize a system for others who will build applications on it or incorporate it in a more complex composite. Other effort aims to improve understanding at the atomic and molecular scale, especially when the phenomena are `simple enough' that there is a good prospect for a quantitative account using first principles theory. Characteristically, a wide range of experimental and theoretical techniques are synthesized to meet these goals. For instance, the papers in this volume include experiments with helium atom scattering (HAS, QHAS) [1], low-energy electron diffraction (LEED) [2,3], scanning tunnelling microscopy (STM) [3], x-ray diffraction [4], and high-resolution electron energy loss spectroscopy (HREELS) [5]. In closely related experiments, thermal desorption (TDS) [6], ellipsometry, [7] and neutron scattering [8] are applied. The balance has evolved over the series of workshops and in the near future one may anticipate a larger component of inelastic and quasi-elastic neutron scattering (INS, QENS) as new facilities are completed.

There is a continuing attempt to use atomic scattering in qualitatively new ways. The last proceedings had a review [9] of efforts to greatly improve the energy resolution of helium beams using spin-echo methods for ³He. The present one has a review [10] of developments of field ionization detectors for He beams that have the prospect of improved detection efficiency and higher spatial resolution for atom interferometers. Since the detection efficiency for most thermal helium beam instruments is only 1/10⁵, there appears to be much room for improvement, but progress has been slow. Another continuing effort is the implementation and interpretation of surface scattering with metastable He beams to probe substrate electronic structures, but the experimental work has had its full share of controversy [11-13].

The focus of these workshops has been on dynamics, but the setting is defined by the spatial structures and surprisingly complex growth scenarios occur for thin films, especially for molecular films. These have been clarified with multi-technique experiments [4]. Even for the adsorption of inert gases, it has been a long process to determine the adsorption sites of minimum energy [2, 14]. In this sub-field, first principles theory now is at the stage of quantitative comparison with experiments [15, 16]. Two examples using ideas of quantum chemistry were presented at the workshop but were published elsewhere [17, 18].

There is work with new materials and more highly controlled materials [19]. In one example, the structure of a quasicrystal was determined by LEED and scanning tunnelling microscopy and the substrate was then used as an adsorbing surface [3]. Even in the Henry's law regime, there are surprises, and a quantitative analysis demonstrates how open the effective surface is. Another example is the use of vicinal surfaces to introduce a controlled population of step defects and then to characterize the excitations of molecules adsorbed at the steps [5].

About half of the papers are primarily theoretical. Further, the standard for LEED experiments in this area is to include quantitative analysis of intensities [2] and experiments on adsorption in the Henry's law regime usually include model building of the adsorption potential energy [3]. For inelastic atomic scattering the situation is decidedly less routine. The contributions here include a review [20] of the development of a mixed quantum-classical theory of inelastic molecular scattering by surfaces and an extension [21] of analytical results for quasi-elastic helium atom scattering with application to low energy and diffusive motions.

One of the theory papers [22] relates to dissipative processes in friction. At first sight this involves a rather different time scale than those for the dynamics emphasized here. However, with increased energy resolution, the surface dynamics experiments get to time scales of several picoseconds while some experiments on sliding friction get down to time scales of a few nanoseconds [23]. Thus, there remains a considerable gap to bridge before the two areas overlap in a useful fashion. Still, this is one likely direction for progress and illustrates both the changing cast of characters in the field and the extensive base of knowledge that is needed for quantitative analysis of both the adsorbate damping and sliding friction. The Surphon workshops appear to have a long life ahead.

L W Bruch

Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA (e-mail: lwbruch@wisc.edu)

Acknowledgment

This work has been partially supported by NSF-DMR0104300.

References

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[2] Diehl R D, Seyller Th, Caragiu M, Leatherman G S, Ferralis N, Pussi K, Kaukasoina P and Lindroos M 2004 J. Phys.: Condens. Matter16 S2839

[3] Trasca R A, Ferralis N, Diehl R D and Cole M W 2004 J. Phys.: Condens. Matter16 S2911

[4] Mo H, Trogish S, Taub H, Ehrlich S N, Volkmann U G, Hansen F Y and Pino M 2004 J. Phys.: Condens. Matter16 S2905

[5] Vattuone L, Savio L and Rocca M 2004 J. Phys.: Condens. Matter16 S2929

[6] Widdra W, Trischberger P, Friess W, Menzel D, Payne S H and Kreuzer H J 1998 Phys. Rev. B 57 4111

[7] Young H S, Meng X F and Hess G B 1993 Phys. Rev. B 4814556

[8] Bienfait M, Asmussen B, Johnson M and Zeppenfeld P 2000 Surf. Sci.460 243

[9] Jardine A P, Ellis J and Allison W 2002 J. Phys.: Condens. Matter14 6173

[10] Doak R B 2004 J. Phys.: Condens. Matter16 S2863

[11] Witte G 2004 J. Phys.: Condens. Matter16 S2937

[12] Trioni M I, Butti G and Bonini N 2004 J. Phys.: Condens. Matter16 S2923

[13] El-Batanouny M 2002 J. Phys.: Condens. Matter14 6281

[14] Gottlieb J M 1990 Phys. Rev. B 42 5377

Müller J E 1990 Phys. Rev. Lett.65 3021

[15] Betancourt A E and Bird D M 2000 J. Phys.: Condens. Matter12 7077

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[17] Bagus P S, Staemmler V and Wöll C 2002Phys. Rev. Lett.89 096104

[18] Phillips J M, Leibsle F, Holder A J and Keith T 2003 Surf. Sci.545 1

[19] Danisman M F, Casilis L, Nickel B and Scoles G 2004 to be published

[20] Moroz I, Ambaye H and Manson J R 2004 J. Phys.: Condens. Matter16 S2953

[21] Vega J L, Guantes R and Miret-Artes S 2004 J. Phys.: Condens. Matter16 S2879

[22] Vanossi A, Santoro G and Bortolani V 2004 J. Phys.: Condens. Matter16 S2895

[23] Dayo A, Alnasrallah W and Krim J 1998 Phys. Rev. Lett.80 1690

Persson B N J 1998 Sliding Friction - Physical Principles and Applications (Berlin: Springer)

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

An overly simplistic but nonetheless useful model is employed to explore basic attributes of field ionization detection. The base cross-sectional detection area of a field ionization tip is set by a centrifugal barrier: molecules impinging at too high a velocity or too large an impact parameter cannot reach the tip and therefore cannot be ionized. For those that do reach the tip, the probability of ionization depends critically on whether they lose sufficient energy on their initial impact to be captured in the polarization field of the tip. Capture is enhanced if the tip radius lies well within the centrifugal barrier and the impinging species is highly polarizable. Scaling relations are developed within the context of this model, allowing recent measurements of field ionization yields to be assessed.

The diffusion and low frequency vibrational motions of atoms and molecules on surfaces can be measured by means of quasielastic helium atom scattering. In this paper, we discuss and investigate different analytical approximations, based on the theory of stochastic processes, to the dynamic structure factor, considering the two motions on an equal footing. Special emphasis is put on the nature of the corresponding lineshapes, explained in terms of the motional narrowing effect. We also discuss the influence of the diffusional and vibrational coupling and several ways of experimentally separating the two types of contributions to the dynamic structure factor. In particular, we propose that the so-called inelastic focusing singularity from atom–surface scattering controls the lineshapes of quasielastic and vibrational peaks.

The hysteretic dynamics of a Frenkel–Kontorova chain subject to irregular substrate potentials and driven by an external dc force is studied both in the underdamped and in the overdamped regime at zero temperature. The choice of a rigid external potential defined by the sum of two sinusoidal functions with different periodicity allows us to simulate microscopic sliding over quasiperiodic and multiple-well periodic substrates. We analyse, for different parameter values of the model, the behaviour of the centre of mass average velocity of the chain as a function of an adiabatic increase and decrease of the applied driving. For small damping coefficients (negligible dissipative forces), at a fixed value of the substrate potential amplitude, the width of the hysteresis region is markedly influenced by the chain stiffness. As expected, in the overdamped dynamical regime no hysteresis is observed. We comment on the nature of the dynamical states displayed during the chain motion at different strengths of the dc driving.

We report on synchrotron x-ray scattering experiments and molecular dynamics simulations of the structure and growth mode of dotriacontane (n-C₃₂H₆₆ or C32) films adsorbed on Ag(111) and SiO₂-coated Si(100) substrates. On the SiO₂ surface, the x-ray measurements confirm a structural model of the solid film inferred from high-resolution ellipsometry measurements in which one or two layers of C32 adsorb with the long axis of the molecule oriented parallel to the interface followed by a monolayer in which the molecules have a perpendicular orientation. At higher C32 coverages, preferentially oriented bulk particles nucleate, consistent with a Stranski–Krastanov growth mode. On the Ag(111) surface, we again observe one or two layers of the 'parallel' film but no evidence of the perpendicular monolayer before nucleation of the preferentially oriented bulk particles. We compare the experimentally observed structures with molecular dynamics simulations of a multilayer film of the homologous C24 molecule.

An interaction potential energy between an adsorbate (Xe and Ar) and the ten-fold Al–Ni–Co quasicrystal is computed by summing over all adsorbate–substrate interatomic interactions. The quasicrystal atoms' coordinates are obtained from LEED experiments, and the Lennard-Jones parameters of Xe–Al, Xe–Ni and Xe–Co are found using semiempirical combining rules. The resulting potential energy function of position is highly corrugated.

Monolayer adsorption of Xe and Ar on the quasicrystal surface is investigated in two cases: (1) in the limit of low coverage (Henry's law regime), and (2) at somewhat larger coverage, when interactions between adatoms are considered through the second virial coefficient, C_AAS. A comparison with adsorption on a flat surface indicates that the corrugation enhances the effect of the Xe–Xe (Ar–Ar) interactions. The theoretical results for the low coverage adsorption regime are compared to experimental (LEED isobar) data.

In this paper a simple approximate approach is used to reproduce the ab initio results for the deexcitation spectral profiles of a metastable helium atom at different distances from a surface. The main quantities of our method are the spin dependent local density of states (LDOS) around the excited helium atom. We show that the extension of the spherical volumes around the atom in which we evaluate the LDOS are the only relevant parameters in this approach and they do not depend on the atom–surface distance.

The sticking probability of C₂H₄ on Ag(210) is studied by the molecular beam technique and by high resolution electron energy loss spectroscopy. At a crystal temperature of 110 K, adsorption occurs initially into a stable, π-bonded, state. At larger coverage a metastable state is populated, too. The sticking probability, S, decreases with increasing translational energy indicating non-activated adsorption. At low coverage and for impact angles from −60° to +60°, S is angle independent, decreasing only as soon as the (100) nanofacets start to be shadowed. At higher coverage, in contrast, S depends on the impact angle, decreasing markedly for grazing incidence onto the (100) nanoterraces. The saturation coverage corresponds to quarter occupancy of the available step sites.

The survival probability of metastable He* atoms scattered from low work function materials depends sensitively on the electronic surface density of states which was utilized to characterize the degree of metallicity of ultrathin alkali metal (AM) films. By combining metastable He-atom scattering (MHAS) with He-atom scattering (HAS) to monitor the film growth, the coverage dependence of the metallization was studied. In this paper the metallization transition obtained for various AM (Na, K, Rb, Cs) and Ba films grown on Cu(100) and GaAs(100) surfaces are compared. It is shown that the onset of metallization observed for K, Cs and Ba on Cu(100) is in close agreement with a critical coverage estimated from a simple 2D Herzfeld model. Moreover, MHAS was also used to investigate the de-metallization of ultrathin AM films upon adsorption of O₂ or CO.

Calculations are carried out and compared with data for the scattering of CH₄ molecules from a LiF(001) surface and for O₂ scattering from Al(111). The theory is a mixed classical-quantum formalism that includes energy and momentum transfers between the surface and projectile for translational and rotational motions as well as internal mode excitation of the projectile molecule. The translational and rotational degrees of freedom couple most strongly to multiphonon excitations of the surface and are treated with classical dynamics. Internal vibrational excitations of the molecules are treated with a semiclassical formalism with extension to arbitrary numbers of modes and arbitrary quantum numbers. Calculations show good agreement for the dependence on incident translational energy, incident beam angle and surface temperature when compared with data for energy-resolved intensity spectra and angular distributions.

We have studied the structure and vibrational dynamics of Cu₃Au(511) using embedded atom method potentials. The relaxation pattern of this surface shows a buckling in all Au-rich chains of about 0.14 Å, which is similar to that found in Cu₃Au(100). The relaxation pattern of this surface is qualitatively similar to that of Cu(511), but is more pronounced in Cu₃Au(511) and extends deep into the bulk. Our calculations show that the local vibrational properties of this vicinal surface is sensitive to the local atomic environment. Surface Au atoms are responsible for a large softening of the force field at the surface, yielding a substantial shift toward low frequencies of the vibrational densities of states associated with Au surface atoms. As a consequence, the excess local free energy of the Au atoms are about 43 meV/atom at 663 K.

In this paper we will present recent results obtained for hydrogen adsorbed on metal oxide surfaces using helium-atom scattering (HAS). The hydrogen adlayers were prepared by exposing clean and well defined metal oxide surfaces to either molecular or atomic hydrogen. The HAS measurements were complemented by LEED and x-ray photoelectron spectroscopy. The results reveal the formation of ordered H (1 × 1) overlayers on rutile TiO₂(110), the polar Zn–ZnO surface and . On exposure to atomic hydrogen did not lead to the formation of an ordered hydrogen adlayer, whereas on the polar O–ZnO surface the hydrogen lifted the (1 × 3) reconstruction seen for the clean surface.

The Debye temperature Θ_D = 411 ± 12 K was determined for a σ-Fe_55.4Cr_44.6 alloy, with average grain size of 27 nm, from Mössbauer spectra recorded in the temperature range of 4–300 K. The result fits well with those found previously for samples of different Cr content, x, with mean grain sizes in the micrometre range. Θ_D was found to increase linearly with x at the rate of 14.6 K/at.%.

Compounds with a Kagomé type lattice are known to exhibit magnetic frustration. Large single crystals of two compounds, Ni₃V₂O₈ and Co₃V₂O₈, which are variants of a Kagomé net lattice, have been grown successfully by the floating zone technique using an optical image furnace. The single crystals are of high quality and exhibit intriguing magnetic properties.

Carbon nanotubes, in which the two-dimensional hexagonal lattice of graphene is transformed into a quasi-one-dimensional lattice by conserving the local bond arrangement, provide several structural parameters for engineering novel physical properties suitable for ultimate miniaturization. Recent interest in nanoscience and nanotechnology has driven a tremendous research activity in carbon nanotubes, which has dealt with a variety of problems and produced a number of new results. Most of the effort has gone into revealing various physical properties of nanotubes and functionalizing them in different ways. This paper covers a narrow region in this enormous research field and reviews only a limited number of recent studies which fit within its scope. First, we examine selected physical properties of bare carbon nanotubes, and then study how the mechanical and electronic properties of different tubes can be modified by radial strain, structural defects and adsorption of foreign atoms and molecules. Magnetization of carbon nanotubes by foreign atom adsorption has been of particular interest. Finally, we discuss specific device models as well as fabricated devices which exploit various properties of carbon nanotubes.

There is renewed emphasis on development of robust solid-state sensors capable of uncooled operation in harsh environments. The sensors should be capable of detecting chemical, gas, biological or radiation releases as well as sending signals to central monitoring locations. We discuss the advances in use of GaN-based solid-state sensors for these applications. AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization-induced two-dimensional electron gas (2DEG). Furthermore, spontaneous and piezoelectric polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors to detect gases, polar liquids and mechanical pressure. AlGaN/GaN HEMT structures have been demonstrated to exhibit large changes in source–drain current upon exposing the gate region to various block co-polymer solutions. Pt-gated GaN Schottky diodes and Sc₂O₃/AlGaN/GaN metal-oxide semiconductor diodes also show large change in forward currents upon exposure to H₂. Of particular interest is detection of ethylene (C₂H₄), which has strong double bonds and hence is difficult to dissociate at modest temperatures. Apart from combustion gas sensing, the AlGaN/GaN heterostructure devices can be used as sensitive detectors of pressure changes. In addition, large changes in source–drain current of the AlGaN/GaN HEMT sensors can be detected upon adsorption of biological species on the semiconductor surface. Finally, the nitrides provide an ideal platform for fabrication of surface acoustic wave (SAW) devices. The GaN-based devices thus appear promising for a wide range of chemical, biological, combustion gas, polar liquid, strain and high temperature pressure-sensing applications. In addition, the sensors are compatible with high bit-rate wireless communication systems that facilitate their use in remote arrays.

Neutron vibrational spectroscopy and neutron diffraction have been used to study the optical hydrogen vibrations in and the structure of ordered niobium hydrides NbH_x and deuterides NbD_x in the concentration range 0.73<x<1.0 and at temperatures between 13 and 250 K. The measurements performed at temperatures above 200 K confirm the established model of the β phase. Neutron diffraction data collected in the regime of the λ phases (T<200 K, 0.73<x<1.0) show that the structure for x<0.85 is made up of microdomains with β phase structure. Neutron vibrational spectra display up to three families of peaks that can be assigned to three different hydrogen sites: an undisturbed tetrahedral site within the microdomains and two strongly disturbed tetrahedral sites within two different kinds of microdomain boundaries. Incommensurately modulated phases probably exist in this concentration range. For hydrogen concentrations around there is evidence for a change of symmetry of the basic cell as well as a change of the modulation at even higher concentrations. In the range of the γ phase at , strong isotopic effects occur. The results give new insights into the phase diagram.

An analysis of a group of slip dislocations coupled with a crack dislocation group in some one- and two-dimensional quasicrystals is given. The extent of the plastic zone and the amount of the dislocation slip have been determined in a very explicit form.

Elastic properties of Mg_(1−x)Al_xB₂ have been studied from first principles. The elastic constants (c₁₁, c₁₂, c₁₃, c₃₃ and c₅₅) have been calculated, in the regime of x = 0 to 0.25. From these calculations the ratio between the bulk modulus and shear modulus (B/G) as well as the ratio between the two directional bulk moduli (B_a/B_c) have been evaluated. Our calculations show that the ratio B_a/B_c decreases monotonically as the aluminium content is increased, whereas the ratio B/G is well below the empirical ductility limit, 1.75, for all concentrations. In addition, we analyse the electronic structure and the nature of the chemical bonding, using the balanced crystal orbital overlap population (BCOOP) (Grechnev et al 2003 J. Phys.: Condens. Matter15 7751) and the charge densities. Our analysis suggests that, while aluminium doping decreases the elastic anisotropy of MgB₂ in the a and c directions, it will not change the brittle behaviour of the material considerably.

The lattice constants and cohesive energies of some possible metastable Cu–W compounds are obtained by ab initio calculation and the formation of a metastable phase at Cu₇₅W₂₅ is predicted for the equilibrium immiscible Cu–W system. The prediction is in agreement with the fact that a metastable hcp phase was indeed observed in the Cu₇₅W₂₅ multilayer films upon ion beam mixing. Furthermore, some of the ab initio calculated properties are used in deriving an n-body Cu–W potential under the embedded atom method. The constructed Cu–W potential is then used to predict the phase stability of the metastable Cu–W phases over the entire composition and the prediction is also supported by some experimental observations.

X-ray diffraction, magnetization, electrical resistivity and thermal expansion measurements were done on a series of Heusler alloys of the form Ni₂MnGa_1−xIn_x 0< x<0.25. X-ray diffraction patterns indicate that the samples possess a highly ordered Heusler alloy L 2₁ type structure. Due to the larger radii of the In ion, the lattice parameters were found to expand with increasing In concentration. The negative volume anomaly () is found to accompany the martensitic transition. With increasing In content, a linear decrease of the martensitic transition and the Curie temperatures were observed. The compositional–transition temperature phase diagram is evaluated. Possible reasons for the structural transformations are discussed.

Li⁺ mobility and structural changes in the aluminosilicate beta-eucryptite have been studied by high-resolution powder neutron diffraction as a function of temperature from 573 to 873 K. A Li split-atom model was used to describe the disorder of Li⁺ ions in the structure. Analysis of the data shows two anomalies around 698 and 758 K. The first anomaly is attributed to an M-point zone boundary transition reported in the literature, which is connected to the appearance of the incommensurate structure. The second anomaly is attributed to a displacive phase transition of the framework and doubling in the a cell parameter.

We apply the functional renormalization group method to the calculation of dynamical properties of zero-dimensional interacting quantum systems. We discuss as case studies the anharmonic oscillator and the single-impurity Anderson model. We truncate the hierarchy of flow equations such that the results are at least correct up to second-order perturbation theory in the coupling. For the anharmonic oscillator, energies and spectra obtained within two different functional renormalization group schemes are compared to numerically exact results, perturbation theory results, and the mean field approximation. Even at large coupling, the results obtained using the functional renormalization group agree quite well with the numerical exact solution. The better of the two schemes is used to calculate spectra of the single-impurity Anderson model, which are then compared to the results from perturbation theory and the numerical renormalization group ones. For small to intermediate couplings the functional renormalization group gives results which are close to the ones obtained using the very accurate numerical renormalization group method. In particular, the low energy scale (Kondo temperature) extracted from the functional renormalization group results shows the expected behaviour.

The magnetic and transport properties of (Pr_1/3Sm_2/3)_2/3A_1/3MnO₃ (A = Ca, Sr and Ba) compounds, prepared by the citrate-gel route, have been investigated. These compounds are found to crystallize in the orthorhombic structure. Charge ordering transport behaviour is indicated only in the Ca-substituted compound. The Sr- and Ba-substituted compounds show a metal–insulator transition and semiconducting-like behaviour, respectively. The magnetoresistance is highest in the Ba-substituted compound. All three samples show irreversibility in magnetization as a function of temperature in zero-field cooled (ZFC) and field cooled (FC) plots. Non-saturating magnetization, even at 5 K and 4 T field, is observed in Ca- as well Ba-substituted compounds.

It is well-known that vibronic interactions can be modelled in terms of an effective Hamiltonian incorporating first and second-order reduction factors (RFs), particularly when analysing the spectroscopic properties. Measurements and calculations of the RFs as a function of the strength of vibronic coupling are therefore of much interest. In this paper, we develop a new general method for determining second-order RFs (soRFs) from the strength of the Jahn–Teller (JT) coupling for systems in which electron orbital degeneracy or pseudo-degeneracy exists. These include in particular the fullerene molecule C₆₀, pseudo-Jahn–Teller molecules and impurity centres in crystals. In order to calculate the important soRFs for intermediate to strong coupling, it is necessary to determine non-Condon corrections to the strong coupling values obtained using the Franck–Condon (FC) approximation. This gives an additional contribution to the nuclear polarizability of the system, thus enabling the electrons to follow the nuclear vibrations. These non-Condon corrections are derived using perturbation theory and are found to be inversely proportional to the square of the JT energy. The validity of the approximation is first tested in the cubic JT system due to its relative simplicity. It is found that the results are closer to those obtained earlier by numerical methods than the analytical FC values alone. Results are then presented that are applicable to C₆₀⁻ anions.

Ultrafast intersubband excitation of electrons in tunnel-coupled wells is studied in respect of its dependence on the structure parameters, the duration of the infrared pump and the detuning frequency. The temporal dependences of the photoinduced carrier concentration and dipole moment are obtained for two cases of transitions: from the single ground state to the tunnel-coupled excited states and from the tunnel-coupled states to the single excited state. The peculiarities of dephasing and population relaxation processes are also taken into account. The nonlinear regime of the response is also considered when the splitting energy between the tunnel-coupled levels is renormalized by the photoexcited electron concentration. The dependences of the period and the amplitude of oscillations on the excitation pulse are presented with a description of the damping of the nonlinear oscillations.

The crystallographic and magnetic structures of Ca₃Mn₂O₇ Ruddlesden–Popper phase have been determined by a combination of neutron and synchrotron x-ray diffraction. Two-phase behaviour observed at room temperature is attributed to an incomplete structural phase transition. The magnetic structure was solved in the Shubnikov group with dominant G-type antiferromagnetic order in the perovskite bilayers. The temperature evolution of the structural and magnetic parameters is presented.

The distribution of magnetization in the ordered Invar alloy Fe₇₂Pt₂₈ has been studied as a function of temperature in the range 100–520 K using polarized neutron diffraction. The temperature range covers both the Invar region and the paramagnetic regime in which the thermal expansion has normal Grüneisen behaviour. The polarized neutron data have been analysed in terms of localized iron 3d and Pt 5d moments allowing local anisotropy of the magnetization distribution on the Fe atoms. The results suggest that the Pt atoms carry a small negative moment () in the magnetically ordered state. The fraction of unpaired electrons with e_g symmetry is approximately 48% and does not change with temperature. This is in contradiction to models of the Invar effect which invoke the thermal population of two states with a different number of e_g and t_2g carriers.

Ultrafine M-type hexagonal ferrites of nominal composition Sr_1−xLa_xFe_12−x Zn_xO₁₉ and Sr_1−xLa_xFe_12−xCo_xO₁₉ with x = 0, 0.1, 0.2, 0.3 and 0.4 were produced by chemical co-precipitation. The phase make-up of the samples was investigated by x-ray diffraction. The analyses show that the hexagonal M-type phase is the main phase in all the samples. Secondary (La,Sr)FeO₃, ZnFe₂O₄ and CoFe₂O₄ phases are also detected, indicating that the La, Zn and Co content in the M-type phase is lower than the nominal content.

A complete Mössbauer analysis of the substitution effects in the M-type phase was made. The results show that the M-type phase contains La³⁺ in the same proportion as Zn²⁺ and La³⁺ in the same proportion as Co²⁺, in the La–Zn and La–Co samples, respectively. The evolution with x of the hyperfine parameters of the components used to fit the contribution of the M-type phase have been interpreted consistently in relation to the substitution effects by means of the comparison between the spectra of the La–Zn and La–Co substituted samples. The results show unambiguously that La³⁺ ions are located in the Sr²⁺ sites, Zn²⁺ ions are located in the 4f₁ sites and Co²⁺ ions are located in both 4f₂ and 2a sites.

The perturbation method is developed to deal with the effective nonlinear dielectric responses of weakly nonlinear graded composites, which consist of the graded inclusion with a linear dielectric function of spatial variables of inclusion material. For Kerr-like nonlinear graded composites, as an example in two dimensions, we have used the perturbation method to solve the boundary value problems of potentials, and studied the effective responses of nonlinear graded composites, where a cylindrical inclusion with linear dielectric function and nonlinear dielectric constant is randomly embedded in a homogeneous host with linear and nonlinear dielectric constants. For the exponential function and the power-law dielectric profiles of cylindrical inclusions, in the dilute limit, we have derived the formulae of effective nonlinear responses of both graded nonlinear composites.