Atomic-scale friction : a scanning probe study on crystalline surfaces

Friction is one of the most fascinating and yet elusive phenomena in physics. Everyday life cannot be imagined in the absence of friction. It allows us to walk, to climb stairs, to sit in a chair, to stop a car, to handle tools. In technological applications friction is the evil of all motion and huge amounts of money are spent annually on energetic and mechanical losses due to friction and wear. Friction is also responsible for natural disasters like earthquakes, landslides and avalanches. Along the centuries mankind has tried to understand and control friction but in spite of the huge volume of experimental knowledge with remarkable technical applications, very little is known about the fundamental, elementary processes taking place on the atomic level at the interface between sliding surfaces. The contact between two apparently flat solids consists in reality of a very large number of micro-protrusions or asperities belonging to both contacting surfaces. Studying and understanding the processes responsible for the occurrence of friction at the buried interface of a single-asperity became the tasks of a new but rapidly expanding science called nanotribology. In the present work we have used a variable-temperature ultra-high vacuum atomic force microscope (VT-UHV-AFM) to investigate the frictional properties, namely the stick-slip behavior, of (100) and (111) crystalline diamond and (001) sodium chloride surfaces. While diamond is a technologically important material and a perfect candidate for an ideal friction experiment, NaCl is well established as a representative model, standard surface for nanotribological investigations. In order to properly simulate the interface with a single asperity at the nanoscale, sharp AFM-tips are used on atomically flat surfaces. The ultra-high vacuum conditions are an essential ingredient for well-defined and reproducible experiments. A hard, stable and sharp AFM-tip termination is of the essence for friction measurements. Therefore, using a hot-filament assisted chemical vapor deposition (HF-CVD) of diamond, a method of growing individual good quality diamond crystallites at the apexes of standard Si AFM tips was demonstrated; the resulting tips showed sharpness, hardness, stability and reliable behavior during friction measurements. We have investigated the atomic-scale friction behaviour between standard silicon nitride tips and diamond-coated tips and a specially prepared hydrogen-terminated (100) diamond sample by means of ultra-high vacuum atomic force microscopy. Tunneling experiments revealed a very flat surface and the typical atomic features (dimers) of a (2x1) surface reconstruction of the hydrogen-terminated (100) diamond sample. When using a diamond-terminated tip, for the first time atomically resolved topography, normal force error signal and lateral force maps are simultaneously obtained and perpendicular domains, hydrogen atomic positions and atomic steps between domains could be observed. This was attributed to a very sharp tip, namely one hydrogen atom-terminated tip, describing a stick-slip movement in two orthogonal directions and causing a dynamic rearrangement of the surface atoms; these results were consistent with an ab-initio electronic structure calculation which reveals the fact that the repulsive interaction between the apex H-atom at the tip and H-atoms at the surface is the essential factor that governs the atomic stick-slip behaviour Similar experiments were carried out in UHV on a (111) crystalline diamond surface with standard silicon nitride and diamond-coated tips. The friction measurements with the same diamond-coated tip on this surface led to atomic resolution: the measured periodicity is consistent with the one of the individual hydrogen atoms of the diamond surface. This reconfirmed the use of an atomically sharp tip and, similar to the results on the (100) diamond surface, we believe that the repulsion between the last H-atom of the tip and the H-atoms of the surface termination is the most important ingredient controlling the complicated two-dimensional atomic scale stick-slip behavior observed experimentally. Finally, atomic-scale friction between a silicon tip and the atomically flat (001) NaCl surface was investigated in ultra-high vacuum at various sample temperatures in the interval from 25 to 300 K. The temperature dependence of measured average friction forces is found to be in good qualitative agreement with theoretical models that consider a thermally-activated discontinuous tip movement during scanning and predict higher friction forces at low temperature. Higher mean friction values observed for two temperature values were attributed to possible changes in the tip apex configuration

Translational-invariant noncommutative gauge theory

A generalized translational invariant noncommutative field theory is analyzed in detail, and a complete description of translational invariant noncommutative structures is worked out. The relevant gauge theory is described, and the planar and nonplanar axial anomalies are obtained.Comment: V1: 23 pages, 4 figures; V2: Section I. improved, References added. Version accepted for publication in PR

Renormalization group-like proof of the universality of the Tutte polynomial for matroids

In this paper we give a new proof of the universality of the Tutte polynomial for matroids. This proof uses appropriate characters of Hopf algebra of matroids, algebra introduced by Schmitt (1994). We show that these Hopf algebra characters are solutions of some differential equations which are of the same type as the differential equations used to describe the renormalization group flow in quantum field theory. This approach allows us to also prove, in a different way, a matroid Tutte polynomial convolution formula published by Kook, Reiner and Stanton (1999). This FPSAC contribution is an extended abstract.Comment: 12 pages, 3 figures, conference proceedings, 25th International Conference on Formal Power Series and Algebraic Combinatorics, Paris, France, June 201

Exorcizing the Landau Ghost in Non Commutative Quantum Field Theory

We show that the simplest non commutative renormalizable field theory, the $\phi^4$ model on four dimensional Moyal space with harmonic potential is asymptotically safe to all orders in perturbation theor

Poincar\'e and sl(2) algebras of order 3

In this paper we initiate a general classification for Lie algebras of order 3 and we give all Lie algebras of order 3 based on $\mathfrak{sl}(2,\mathbb C)$ and $\mathfrak{iso}(1,3)$ the Poincar\'e algebra in four-dimensions. We then set the basis of the theory of the deformations (in the Gerstenhaber sense) and contractions for Lie algebras of order 3.Comment: Title and presentation change

lncRNA-dependent mechanisms of androgen-receptor-regulated gene activation programs.

Although recent studies have indicated roles of long non-coding RNAs (lncRNAs) in physiological aspects of cell-type determination and tissue homeostasis, their potential involvement in regulated gene transcription programs remains rather poorly understood. The androgen receptor regulates a large repertoire of genes central to the identity and behaviour of prostate cancer cells, and functions in a ligand-independent fashion in many prostate cancers when they become hormone refractory after initial androgen deprivation therapy. Here we report that two lncRNAs highly overexpressed in aggressive prostate cancer, PRNCR1 (also known as PCAT8) and PCGEM1, bind successively to the androgen receptor and strongly enhance both ligand-dependent and ligand-independent androgen-receptor-mediated gene activation programs and proliferation in prostate cancer cells. Binding of PRNCR1 to the carboxy-terminally acetylated androgen receptor on enhancers and its association with DOT1L appear to be required for recruitment of the second lncRNA, PCGEM1, to the androgen receptor amino terminus that is methylated by DOT1L. Unexpectedly, recognition of specific protein marks by PCGEM1-recruited pygopus 2 PHD domain enhances selective looping of androgen-receptor-bound enhancers to target gene promoters in these cells. In 'resistant' prostate cancer cells, these overexpressed lncRNAs can interact with, and are required for, the robust activation of both truncated and full-length androgen receptor, causing ligand-independent activation of the androgen receptor transcriptional program and cell proliferation. Conditionally expressed short hairpin RNA targeting these lncRNAs in castration-resistant prostate cancer cell lines strongly suppressed tumour xenograft growth in vivo. Together, these results indicate that these overexpressed lncRNAs can potentially serve as a required component of castration-resistance in prostatic tumours

Super-Group Field Cosmology

In this paper we construct a model for group field cosmology. The classical equations of motion for the non-interactive part of this model generate the Hamiltonian constraint of loop quantum gravity for a homogeneous isotropic universe filled with a scalar matter field. The interactions represent topology changing processes that occurs due to joining and splitting of universes. These universes in the multiverse are assumed to obey both bosonic and fermionic statistics, and so a supersymmetric multiverse is constructed using superspace formalism. We also introduce gauge symmetry in this model. The supersymmetry and gauge symmetry are introduced at the level of third quantized fields, and not the second quantized ones. This is the first time that supersymmetry has been discussed at the level of third quantized fields.Comment: 14 pages, 0 figures, accepted for publication in Class. Quant. Gra