Lattice Relaxation in Solid He-4: Effect on Dynamics of He-3 Impurities

We present a new simplified derivation of the effect of lattice relaxation that accompanies the quantum tunneling of He-3 impurities in solid He-4 on the nuclear spin-lattice relaxation of the He-3 impurities for very low impurity concentrations. As a result of the larger zero point motion of the He-3 impurity compared to the He-4 atoms, a significant lattice distortion accompanies the impurity as it moves through the lattice and the dynamics of the impurity depends on both the interaction energy between two He-3 atoms and on the relaxation of the lattice for the tunneling impurity. Using a phenomenological model for the lattice relaxation we compare the nuclear spin-lattice relaxation rates observed at low temperatures with the dependence on temperature expected for a He-4 lattice relaxation comparable to that observed by Beamish et al. (Phys. Rev. Lett. 96:195304, 2006).

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

Submonolayer epitaxial growth is obtained by the deposition of less than a complete layer of atoms on a single crystal surface. It is of fundamental interest for industrial applications (e.g. in the semiconductor industry) as well as from the point of view of basic research. For example, it is known that nanometer-sized atomic structures (nanostructures) exhibit remarkable physical and chemical properties which can differ greatly from those of bulk matter. In order to investigate these properties, it is often necessary to create large quantities of well defined and possibly spatially ordered nanostructures. One way to achieve this result is self-organized growth where one deposits atoms on a clean crystalline surface and lets the growth process evolve freely. Here, nanostructures result from the atomic diffusion and aggregation processes taking place at the surface. Understanding the exact nature of these processes is of ongoing interest in the field of nanostructure growth. In this thesis we report about submonolayer growth experiments of the ferromagnetic transition metal cobalt. Cobalt was chosen because it exhibits remarkable magnetic properties. These experiments were performed in an ultra-high vacuum chamber and molecular beam epitaxy was used to grow the nanostructures. Observations of the surface were made using a variable-temperature scanning tunneling microscope (STM). In order to get a better understanding of the atomic processes happening at the surface, we developed two adapted computational simulation methods. Kinetic Monte-Carlo (KMC) simulations were used to get an atomistic picture of the surface while mean field rate equations were integrated numerically to yield cluster densities. We study the growth of cobalt on three different surfaces. By depositing Co on a clean Pt(111) crystal surface, we observe that the platinum surface reconstructs by forming star-shaped partial dislocations for sample temperatures above 180 K (-93°C). We also observe that island densities deviate from predictions of all known models towards higher values for these same temperatures. By simulation we are able to show that insertion of Co into the topmost platinum layer creates a repulsive network of dislocations. We show that these dislocations act as diffusion inhibiting barriers and thus influence the island density by constraining the free movement of atoms at the surface. We also show that the Co dimer and trimer diffuse on Pt(111) before dissociating and are able to extract corresponding activation barriers. We also study the deposition of Co on a Ru(0001) surface which shows a more conventional temperature dependence. By comparing simulation and experiment we extract all relevant diffusion activation barriers and show that at high temperatures the Co dimer and trimer dissociate. Finally, we investigate the growth of Co on the hexagonal boron nitride superlattice which forms on a Rh(111) surface. On this surface Co clusters form three-dimensional hemispherical dots. We show how the substrate corrugation influences diffusion of Co atoms and that clusters up to the size of the pentamer can diffuse. By simulation, we also extract the relevant migrations and desorption barriers.

EPFL2008

Etude par STM de la déposition d'agrégats d'or sur TiO2 et mesure de l'émission électronique secondaire induite par l'impact d'agrégats sur une surface

Pierre Convers

This thesis work focuses on impact and diffusion processes as well as equilibrium positions of a metal deposited on a surface. The metal is deposited in the form of clusters containing n atoms in a controlled way. Gold or silver clusters cations (Au+n and Ag+n ) are used. Their size (n ∈ [1, 9]), charge state and deposition energy (E = 10 - 4500 eV) are well defined. The surface being at room temperature during the deposition, can be annealed and transferred in a scanning tunnelling microscope (STM) after deposition. The variable temperature microscope was developed during this thesis. The approach of the tip is carried out by an axial motor, which renders the microscope very stable. The first part of this work treats of the evolution of gold deposited on a TiO2(110) surface. Position and size of gold islands created by cluster or atomic depositions are determined by STM. Compared to atomic deposition at the same energy (10 < E < 100 eV), the cluster deposition produces smaller islands with a large islands density after annealing at 800 K. Regardless the deposition conditions, gold atoms diffuse on the surface already at 300 K. The impact energy is the key parameter to reduce the diffusion by cluster or atomic implantation or defect creation. Upon a high energy deposition, part of the deposited gold is invisible to the STM. The fate of these gold atoms is unclear : they are either buried under the surface or ejected in vacuum at the impact. The control of the islands size should permit the creation of a stable system with an optimum catalytic activity (i. e. CO combustion). Research groups have shown that this activity is strongly influenced by the islands size. It is equal to zero if the diameter is greater than 5 nm. The second part of this work is devoted to the electron emission γ induced by the impact of silver clusters Ag+n on a Pt and HOPG surface. Measurements are made by varying the impact energy (and so the speed v) of size-selected clusters on a defined surface. Impact velocity ranges from 104 to 105 m/s, which is lower than the classical threshold. Nevertheless every cluster size produces an electron emission. A potential emission (γ(v=0) ≠ 0) is observed for the monomer on both surfaces, which is caused by excited ions in a metastable state. The γ(v) curves are increasing, and no mesurable oscillation are observed. This does not confirm earlier work by Meiwes-Broer et al. These authors found oscillations in γ on similar systems relating them to the electronic structure of the clusters and the surface. A model based on heating of the electron gas is developed. This model gives good agreements with the γ(v) curves. The electronic temperature is estimated to 3000-8000 K. Similar behavior on both surfaces (Pt and HOPG) depending of the cluster size is shown. This behavior is probably related to the geometrical structure of the clusters. Finally, a more pronounced molecular effect is observed for the HOPG : the number of electrons emitted by the impact of a Ag+n is up to 7 times higher than the number of electrons emitted by impacts of n independent atoms. This value is smaller than 2 for the Pt surface.

EPFL2005

Intermediate temperature grain boundary phenomena in copper alloys

Doris Empl

Copper and copper alloys typically experience a sharp loss in ductility at intermediate temperature, induced by grain boundary damage and fracture processes. The susceptibility of the material to intermediate temperature embrittlement is, as a result, strongly affected by the presence, along the grain boundary, of segregated atoms or low-melting phase inclusions, the former potentially weakening atomic bonding and the second causing liquid metal embrittlement along the boundaries. This thesis is an exploration of underlying phenomena that govern the intermediate temperature embrittlement of Cu-Pb alloys, addressing in turn capillary phenomena and then their relation to the tensile strength and ductility of leaded copper alloys. The dihedral angle shown by intergranular lead inclusions in Cu-1Pb alloys is measured varying the purity of the metal and the temperature. Several measurement methods are used and compared, namely classical 2D methods based on metallurgical cross section analysis, and a 3D stereoscopic method that yields the true three-dimensional angle value for individual inclusions straddling a flat grain boundary (first presented by Felberbaum et al., J. Mat. Sci., vol. 40 (2005) p. 3121). Data confirm and extend earlier measurements using the newer 3D method. It is shown that a discrepancy found between literature data and stereoscopic 3D dihedral angle measurements is not caused by impurity effects, but the data indicate that its origin lies in a difference in average dihedral angle values measured between inclusions straddling two grains, and values found at inclusions located where three or more grains meet. The method is then extended towards the simultaneous measurement of relative grain boundary energy and all five macroscopic grain boundary degrees of freedom in a copper polycrystal containing quenched lead inclusions. Data are compared with a model for grain boundary energy in pure fcc metals. The comparison suggests (i) that relaxation can lower the grain boundary energy significantly when equal numbers of broken bonds are present on either side of the grain boundary and (ii) that lead segregation to grain boundaries affects the orientation-dependence of large-angle grain boundary energies in copper. A study of the tensile strength of two Cu-1Pb alloys of different purity and of different microstructure tested at 400 °C and at a strain rate of 10 s-1 is then presented. Data show that the alloy fracture stress is consistent with a Griffith-type fracture criterion that results from assimilation of the inclusions to a void. Measured fracture stresses suggest a 25-fold increase in fracture energy dissipation over surface tension contributions, likely caused by plastic deformation of the ductile copper phase. Tensile tests conducted at 400 °C and at a strain rate of 10-2 s-1 are then presented for a Cu-Ni-Sn-Pb alloy, which exhibits high mechanical strength at room temperature coupled with good machinability. Additions to this alloy of less than 1 wt% are investigated, namely Al, Mn, Zr, B or P. It is found that, unlike the other alloying additions explored, B and P improve the strength and ductility of the leaded alloy and also that these form second phase particles in the alloy; it is proposed that the improvement results from a capillary and mechanical interaction between these particles and molten lead inclusions in the alloy, consistent with the theory advanced to explain results found for the Cu-1Pb alloy. The thesis concludes with a preliminary exploration of grain boundary engineering strategies towards the improvement of the 400 °C strength of Cu-1%Pb, showing that marginal improvements in both strength and ductility can be achieved.

EPFL2009