Spin dynamics above the curie temperature studied by neutron scattering

Neutron scattering can in principle give information about magnetic fluctuations over the entire atomic space and time domain. The weakness of the neutron-matter interaction renders this information undistorted by the neutron probe, but at the same time puts intensity limitations on the method. A considerable number of studies on the magnetism of 3d metals have been performed at some of the larger reactor laboratories. In the regions of overlap the experimental results from the different laboratories are consistent, but the interpretations are along different lines. Among the controversial issues are itinerancy versus localization, the degree of order above T_c. In our talk we shall give an introduction to the neutron scattering method, including some of the sophisticated polarized beam methods. In the rest of the talk we shall review recent experimental results and some of the theoretical models used in their interpretation.     

Structural and electronic properties of hexa-adamantyl-hexa-phenylbenzene molecules studied by low temperature scanning tunneling microscopy

The electronic and structural properties of large molecules composed of a central hexa-phenylbenzene core surrounded by six adamantyl groups (Ad₆HPB) adsorbed on Ag(111) have been investigated using low temperature scanning tunneling microscopy (STM), adsorption and image calculations. On large 2D domains, the molecular organization displays two lattices. In one of them, molecules are slightly nested as a result of their relative flexibility during packing. These compounds also exhibit contrast variations in terms of used bias voltage in the self-assembled domains as well as in single molecule observations. This is attributed to the peculiar electronic properties of Ad₆HPB and to the role of peripheral groups.     

Translational and rotational parameters obtained from neutron scattering studies of near-spherical molecules

It is shown that translational and rotational parameters obtained from slow neutron scattering experiments on systems of near-spherical molecules essentially agree with the results derived from computer-simulated many-body studies based on rough-sphere particles.     

Effective long-range attraction between protein molecules in solutions studied by small angle neutron scattering

Small angle neutron scattering intensity distributions taken from cytochrome C and lysozyme protein solutions show a rising intensity at a very small wave vector Q, which can be interpreted in terms of the presence of a weak long-range attraction between protein molecules. This interaction has a range several times that of the diameter of the protein molecule, much greater than the range of the screened electrostatic repulsion. We show evidence that this long-range attraction is closely related to the type of anion present and ion concentration in the solution.     

Proton spin conversion of coupled methyl groups in lithium acetate studied by inelastic neutron scattering

The temperature dependence of the tunneling spectrum of methyl groups in lithium acetate dihydrate has been studied in the temperature range between 1.2 and 8.0 K by inelastic neutron scattering. The results unambiguously prove that it is to a first order approximation correct to describe the tunneling motions by a model of coupled CH₃ pairs which are isolated from each other. However, from the fact that the tunneling frequencies shift to higher values with decreasing spin temperature, we conclude that coupling effects are important not only between nearest neighbour CH₃ groups. Quantitatively we can describe the observations by a model of coupled pairs with a fixed value for the interaction potentialW ₃ and a variable single particle potentialV ₃ which depends linearly on the concentration of the spin symmetry species.     

Surface dynamics studied by time-dependent tunneling current

Scanning tunneling microscopy (STM) is not only an excellent tool for the study of static geometric structures and electronic structures of surfaces due to its high spatial and energy resolution, but also a powerful tool for the study of surface dynamic behaviors, including surface diffusion, molecular rotation, and surface chemical reactions. Because of the limitation of the scanning speed, the video-STM technique cannot study the fast dynamic processes. Alternatively, a time-dependent tunneling current, I-t curve, method is employed in the research of fast dynamic processes. Usually, this method can detect about 1000 times faster dynamic processes than the traditional video-STM method. When placing the STM tip over a certain interesting position on the sample surface, the changing of tunneling current induced by the surface dynamic phenomena can be recorded as a function of time. In this article, we review the applications of the time-dependent tunneling current method to the studies of surface dynamic phenomena in recent years, especially on surface diffusion, molecular rotation, molecular switching, and chemical reaction.     

The interaction of ions and easily ionized species with oxide surfaces as studied by tunneling spectroscopy

The thin film properties of alumina and magnesia, the substrates most frequently used in tunneling spectroscopy, are reviewed. The composite barriers, Al₂O₃/SiHO_x and Al_xSO_y are also discussed. Tunneling studies provide information about absorbate-surface interactions; weak counterion adsorption effects through very strong direct chemical modifications can be studied. Tunneling studies of anionic transition metal complexes adsorbed on alumina provide significantly more information about the nature of the surface species than corresponding IR studies. The adsorption of TCNE on alumina from solution is used to exemplify how tunneling spectroscopy may be integrated into a manifold of conventional techniques (IR, ESR, and elution studies in this case).     

The temperature dependence of rotational tunneling

In the last few years tunneling transitions have been observed for the highly symmetric groups CH₄, CD₄, NH ₄ ⁺ , and CH₃ rotating in various environments. Typically the tunneling lines shift to lower energies with increasing temperatures. In this paper the shift of the tunneling energy is calculated in a microscopic approach to the problem. The coupling of the rotating groups to the lattice modes is studied in two stages. First the rotating group is coupled to a single oscillator, then to the modes of a Debye crystal. The first calculation leads to a set of discrete tunneling lines with an energy that diminishes as the oscillator is excited into higher levels. The second approach yields a single tunneling line shifted down-wards with increasing phonon population. The shift is proportional toT ⁴. The calculation explains the energy shift of the tunneling lines with reasonable values for the coupling parameters. In some cases also a broadening has been observed which does not follow from our calculations.     

Metal–insulator-transition studied by single-electron tunneling

We present measurements of single-electron tunneling in a vertical GaAs/AlGaAs double-barrier resonant-tunneling device with a low emitter doping. The transport spectrum of our sample exhibits a series of differential conductance peaks which experience an exponential shift to higher voltages with magnetic fields beyond a critical magnetic field. We attribute this effect to a metal-insulator transition in our device. A detailed analysis of the temperature-dependence of this effect is shown.     

Methyl rotation in acetamide: the transition from quantum mechanical tunneling to classical reorientation studied by inelastic neutron scattering

Quasielastic and inelastic incoherent neutron scattering has been used to study in detail the transition from quantum mechanical tunneling motion to classical reorientation of the methyl groups in rhombohedral acetamide CH₃CONH₂. The temperature dependence of the low temperature quasielastic and inelastic scattering due toE ^a E ^b and AE transitions of the tunneling methyl groups has been investigated with eV resolution and — together with the higher temperature quasielastic scattering-compared with theoretical predictions. Microscopic theories are capable to describe most of the experimental observations at low temperatures. A heuristic theoretical approach accounts well for the high temperature results.     

Magnetic properties of superconductors studied by neutron depolarization

A new method is described which allows the study of the magnetic properties of superconductors. Measurements on various niobium and lead samples were taken to study the influence of surface conditions and lattice defects on the formation of the mixed and intermediate states. A strong dependence of flux penetration on surface conditions is found. AboveH _C1 the results indicate the formation of a disturbed flux line lattice. The influence of external fields present when the superconductor is cooled throughT _c is discussed and evidence for the intermediate state of type II superconductors is presented. Finally a simple model is used to evaluate the mean angle of deviation from the ideal lattice.     

Atomic-scale surface science phenomena studied by scanning tunneling microscopy

Following the development of the scanning tunneling microscope (STM), the technique has become a very powerful and important tool for the field of surface science, since it provides direct real-space imaging of single atoms, molecules and adsorbate structures on surfaces. From a fundamental perspective, the STM has changed many basic conceptions about surfaces, and paved the way for a markedly better understanding of atomic-scale phenomena on surfaces, in particular in elucidating the importance of local bonding geometries, defects and resolving non-periodic structures and complex co-existing phases. The so-called “surface science approach”, where a complex system is reduced to its basic components and studied under well-controlled conditions, has been used successfully in combination with STM to study various fundamental phenomena relevant to the properties of surfaces in technological applications such as heterogeneous catalysis, tribology, sensors or medical implants. In this tribute edition to Gerhard Ertl, we highlight a few examples from the STM group at the University of Aarhus, where STM studies have revealed the unique role of surface defects for the stability and dispersion of Au nanoclusters on TiO₂, the nature of the catalytically active edge sites on MoS₂ nanoclusters and the catalytic properties of Au/Ni or Ag/Ni surfaces. Finally, we briefly review how reaction between complex organic molecules can be used to device new methods for self-organisation of molecular surface structures joined by comparatively strong covalent bonds.     

Spin relaxation rates in CuFe measured by neutron scattering

The absolute value and the temperature dependence of the spin relaxation rate of the Kondo system $\text{Cu}$Fe has been deduced from the magnetic scattering of unpolarized neutrons with energy analysis.     

Spin dynamics in conducting polymers studied by μSR

We report studies of spin dynamics in the conducting polymers polyaniline and polypyrrole using both μ⁺SR and μ^-SR techniques. These measurements reveal characteristic field dependences and cutoff frequencies for the muon spin relaxation which can be related to the spin diffusion process. Clear evidence is seen for increased spin localisation at low temperatures where a crossover occurs from two or three dimensional spin diffusion to a one dimensional diffusion regime. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin dynamics in CuGeO3 studied by muon spin rotation

We report a muon spin rotation (SR) study of the magnetic properties of the Cu²⁺ quasi-one-dimensional CuGeO₃ system and its lightly-doped derivative Cu_0.97Zn_0.03GeO₃. Susceptibility measurements on CuGeO₃ show a sudden change in the vicinity of 14 K that has been interpreted before as a magnetic transition to a spin-Peierls state. SR shows no evidence of spin freezing below 14 K, implying that the transition is to a magnetic state with no static (random or ordered) electronic moments. A modest slowing down of the electronic spin dynamics is also identified at this temperature. Similarly, no evidence of a transition to a static magnetic state is found for Cu_0.97Zn_0.03GeO₃ whose susceptibility shows hysteretic behaviour between zero-field and field cooled measurements at 4 K, previously ascribed to spinglass-like behaviour. Given the nature of the muon spin as a local magnetic probe, the present results necessitate a re-interpretation of the origin of the susceptibility anomaly observed in the doped system.