In many applications like photovoltaics, fuel cells, batteries, or interconnects in integrated circuits carbon nanotubes (CNTs) have the role of charge transport electrodes. The building of such devices requires an in situ growth of CNTs at temperatures where the structure or chemical composition of the functional materials is unaltered. We report that in a chemical vapor deposition process involving an oxidative dehydrogenation reaction of C2H2 with CO2 growth temperatures below 400 degrees C are achieved. Furthermore, the growth can be performed on versatile materials ranging from metals through oxides to organic materials.

We investigate the conditions required for general spin systems with frustration and disorder to display self-organized criticality, a property which so far has been established only for the fully-connected infiniterange Sherrington-Kirkpatrick Ising spin-glass model [Phys. Rev. Lett. 83, 1034]. Here we study both avalanche and magnetization jump distributions triggered by an external magnetic field, as well as internal field distributions in the short-range Edwards-Anderson Ising spin glass for various space dimensions between 2 and 8, as well as the fixed-connectivity mean-field Viana-Bray model. Our numerical results, obtained on systems of unprecedented size, demonstrate that self-organized criticality is recovered only in the strict limit of a diverging number of neighbors, and is not a generic property of spin-glass models in finite space dimensions. PACS numbers: 75.50.Lk, 75.40.Mg, 05.50.+q, Self-organized criticality (SOC) refers to the tendency of large dissipative systems to drive themselves into a scaleinvariant critical state without any special parameter tuning [1,2]. These phenomena are of crucial importance because fractal objects displaying SOC are found everywhere [3], e.g., in earthquakes, in the structure of dried-out rivers, in the meandering of sea coasts, or in the structure of galactic clusters. Understanding its origin, however, represents a major unresolved puzzle because in most equilibrium systems critical behavior featuring scale-free (fractal) patterns is found only at isolated critical points and is not a generic feature across phase diagrams.Pioneering work in the 1980s provided insights into the possible origin of SOC by identifying a few theoretical examples that display it. The "sandpile" [4] and forest-fire models [5] are hallmark examples of dynamical systems that exhibit SOC. However, these models feature ad hoc dynamical rules, without showing how these can be obtained from an underlying Hamiltonian. Major questions thus remain: Can one obtain SOC from a Hamiltonian system, beyond invasion percolation [6,7]? Is this behavior a feature of high-dimensional models, models with a diverging number of neighbors and/or long-range interactions, or is it a generic property of a broad class of systems?Work in the 1990s offered a glint of hope. The first Hamiltonian model displaying SOC without any parameter tuning was studied in detail by Pazmandi et al. [8]: the infinite-range fully connected Sherrington-Kirkpatrick (SK) model [9]. Outof-equilibrium avalanches at zero temperature (T = 0) triggered by varying the magnetic field were numerically studied along the hysteresis loop. A distinct power-law behavior in the distribution of spin avalanches, as well as of the magnetization jumps, was established, i.e., SOC.The possible existence of SOC was also tested in several finite-dimensional models, but in all these cases, at least one parameter has to be tuned. The best-studied such model is the random-field Ising model where ferromagnetic Ising spins are coupled to a random field of ave...

In the accepted mechanisms of carbon nanotube (CNT) growth by catalytic chemical vapor deposition (CCVD), the catalyst support is falsely considered as a passive material whose only role is to prevent catalytic particles from coarsening. The chemical changes that occur to the carbon source molecules on the surface are mainly overlooked. Here, we demonstrate the strong influence of the support on the growth of CNTs and show that it can be tuned by controlling the acid-base character of the support surface. This finding largely clarifies the CCVD growth mechanism. The CNTs' growth stems from the support where the presence of basic sites catalyzes the aromatization and reduces the complexity of CNT precursor molecules. On basic supports, the growth is activated and CNTs are more than 1000 times longer than those produced on acidic supports. These results could be the bedrock of future development of more efficient growth of CNTs on surfaces of functional materials. Finally, the modification of the aciditiy of the catalyst support during the super growth process is also discussed.

The nature of ordering in dilute dipolar interacting systems dates back to the work of Debye and is one of the most basic, oldest and as-of-yet unsettled problems in magnetism. While spin-glass order is readily observed in several RKKY-interacting systems, dipolar spin glasses are the subject of controversy and ongoing scrutiny, e.g., in LiHo x Y 1−x F 4 , a rare-earth randomly diluted uniaxial (Ising) dipolar system. In particular, it is unclear if the spin-glass phase in these paradigmatic materials persists in the limit of zero concentration or not. We study an effective model of LiHo x Y 1−x F 4 using large-scale Monte Carlo simulations that combine parallel tempering with a special cluster algorithm tailored to overcome the numerical difficulties that occur at extreme dilutions. We find a paramagnetic to spin-glass phase transition for all Ho þ ion concentrations down to the smallest concentration numerically accessible, 0.1%, and including Ho þ ion concentrations that coincide with those studied experimentally up to 16.7%. Our results suggest that randomly diluted dipolar Ising systems have a spin-glass phase in the limit of vanishing dipole concentration, with a critical temperature vanishing linearly with concentration. The agreement of our results with mean-field theory testifies to the irrelevance of fluctuations in interactions strengths, albeit being strong at small concentrations, to the nature of the low-temperature phase and the functional form of the critical temperature of dilute anisotropic dipolar systems. Deviations from linearity in experimental results at the lowest concentrations are discussed.