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Carbon nanotubes tend to collapse when their diameters exceed a certain threshold, or when a sufficiently large external pressure is applied on their walls. Their radial stability of tubes has been studied in each of these cases, however a general theory able to predict collapse is still lacking. Here, we propose a simple model predicting stability limits as a function of the tube diameter, the number of walls and the pressure. The model is supported by atomistic simulations, experiments, and is used to plot collapse phase diagrams. We have identified the most stable carbon nanotube, which can support a maximum pressure of 18 GPa before collapsing. The latter was identified as a multiwall tube with an internal tube diameter of 12nm and 30 walls. This maximum pressure is lowered depending on the internal tube diameter and the number of walls. We then identify a tube diameter domain in which the radial mechanical stability can be treated as equivalent to macroscopic tubes, known to be described by the canonical L\'evy-Carrier law. This multiscale behavior is shown to be in good agreement with experiments based on O-ring gaskets collapse, proposed as a simple macroscopic parallel to nanotubes in this domain., Comment: 32 pages, 9 figures

Euler solved the problem of the collapse of tall thin columns under unexpectedly small loads in 1744. The analogous problem of the collapse of circular elastic rings or tubes under external pressure was mathematically intractable and only fully solved recently. In the context of carbon nanotubes, an additional phenomenon was found experimentally and in atomistic simulations but not explained: the collapse pressure of smaller diameter tubes deviates below the continuum mechanics solution [Torres-Dias et al., Carbon 123, 145 (2017)]. Here, this deviation is shown to occur in discretized straight columns and it is fully explained in terms of the phonon dispersion curve. This reveals an unexpected link between the static mechanical properties of discrete systems and their dynamics described through dispersion curves., Comment: 4 pages, 3 Figures, draft in preparation

Density functional theory has been employed to study graphene on the (111), (100) and (110) surfaces of silicon (Si) substrates. There are several interesting findings. First, carbon atoms in graphene form covalent bonds with Si atoms, when placed close enough on Si (111) and (100) surfaces, but not on the (110) surface. The presence of a Si (111) surface shifts the Fermi level of graphene into its conduction band, resulting in an increase of the work function by 0.29 eV and of the electron density by three orders of magnitude. The carrier density of graphene can also be increased by eighty times on a Si (100) substrate without doping, due to the modification of the density of states near the Dirac point. No interfacial covalent bond can be formed on Si (110). These striking effects that different orientations of a silicon substrate can have on the properties of graphene are related to the surface density of the silicon surface. Applying the results to a real device of a specific orientation requires further consideration of surface reconstructions, lattice mismatch, temperature, and environmental effects., Comment: 8 pages, 4 figures

Previous Raman measurements on supported graphene under high pressure reported a very different shift rate of in-plane phonon frequency of graphene (16 cm$^{-1}$GPa$^{-1}$) from graphite (4.7 cm$^{-1}$GPa$^{-1}$), implying very different in-plane anharmonicity that graphene gets stiffer than graphite in-plane under the same pressure. It was suggested that it could be due to the adhesion of graphene to substrates. We have therefore performed high pressure Raman measurements on unsupported graphene and we find a similar in-plane stiffness and anharmonicity of graphene (5.4 cm$^{-1}$GPa$^{-1}$) to graphite. On the other hand, the out-of-plane stiffness of graphene is hard to define, due to the 2D nature of graphene. However, we estimate a similar out-of-plane stiffness of graphene (1.4$\pm$295 GPa) to that of graphite (38.7$\pm$7 GPa), by measuring its effect on the shift of the in-plane phonon frequency with pressure., Comment: 5 pages, 5 figures

The lubricating ability of graphite largely depends on the environmental humidity, essentially the amount of water in between its layers. In general, intercalated molecules in layered materials modify their extraordinary properties by interacting with the layers. To understand the interaction of intercalated water molecules with graphene layers, we performed Raman measurements on bi-layer graphene at various humidity levels and observed an additional peak close to that of the low-frequency layer breathing mode between two graphene layers. The additional peak is attributed to the vibration between an intercalated water layer and the graphene layers. We further propose that the monolayer coverage of water increases between bilayer graphene with increasing environmental humidity while the interaction between the water layer and graphene layers remains approximately unchanged, until too much water is intercalated to keep the monolayer structure, at just over 50\% relative humidity. Notably, the results suggest that unexpectedly humidity could be an important factor affecting the properties of layered materials, as it significantly modifies the interlayer interaction., Comment: 6 pages, 5 figures

The Hall-Petch effect has been described for the past sixty years as a dependence of the strength of polycrystalline metals on the inverse square-root of grain size d. The value of the coefficient of the dependence has been the subject of discussion throughout. Here, we find what factors in the experiments determine its values. A meta-analysis using maximum-likelihood methods is reported of the literature values of the coefficient in sixty-one datasets. Weak dependence is found on composition and bulk strength. Clear dependence is found on the elastic anisotropy of the different metals and on their stacking fault energies. Surprisingly, no dependence is found on plastic strain, and the strongest dependence is found on the average grain size of each study. Combining these effects accounts for the reported values of about 80% of the sixty-one coefficients. The grain-size dependence and the bulk strength dependence indicate that the Hall-Petch coefficient is an artefact arising from incorrect fitting of the data. Moreover, the grain-size dependence implies a minimum strength described by a simple inverse 1/d or a lnd/d function, which arises theoretically from considerations of dislocation curvature., Comment: 19 pages, 5 Figures, 3 Tables

The MAJORANA DEMONSTRATOR is an array of point-contact Ge detectors fabricated from Ge isotopically enriched to 88% in Ge-76 to search for neutrinoless double beta decay. The processing of Ge for germanium detectors is a well-known technology. However, because of the high cost of Ge enriched in Ge-76, special procedures were required to maximize the yield of detector mass and to minimize exposure to cosmic rays. These procedures include careful accounting for the material; shielding it to reduce cosmogenic generation of radioactive isotopes; and development of special reprocessing techniques for contaminated solid germanium, shavings, grindings, acid etchant and cutting fluids from detector fabrication. Processing procedures were developed that resulted in a total yield in detector mass of 70%. However, none of the acid-etch solution and only 50% of the cutting fluids from detector fabrication were reprocessed. Had they been processed, the projections for the recovery yield would be between 80 -- 85%. Maximizing yield is critical to justify a possible future ton-scale experiment. A process for recovery of germanium from the acid-etch solution was developed with yield of about 90%. All material was shielded or stored underground whenever possible to minimize the formation of Ge-68 by cosmic rays, which contributes background in the double-beta decay region of interest and cannot be removed by zone refinement and crystal growth. Formation of Ge-68 was reduced by a significant factor over that in natural abundance detectors not protected from cosmic rays., Comment: Submitted to NIM-A

Layered materials have huge potential in various applications due to their extraordinary properties. To determine the interlayer interaction (or equivalently the layer spacing under different perturbations) is of critical importance. In this letter, we focus on one of the most prominent layered materials, graphite, and theoretically quantify the relationship between its interlayer spacing and the vibrational frequencies of its layer breathing and shear modes, which are measures of the interlayer interaction. The method used here to determine the interlayer interaction can be further applied to other layered materials., Comment: 5 pages, 3 figures

A high power pulsed laser system has been installed on the high magnetic field muon spectrometer (HiFi) at the ISIS pulsed neutron and muon source, situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade enables one to perform light-pump muon-probe experiments under a high magnetic field, which opens new applications of muon spin spectroscopy. In this report we give an overview of the principle of the HiFi Laser system, and describe the newly developed techniques and devices that enable precisely controlled photoexcitation of samples in the muon instrument. A demonstration experiment illustrates the potential of this unique combination of the photoexcited system and avoided level crossing technique., Comment: 12 pages, 7 figures, and 2 tables

Stacking graphene sheets forms graphite. Two in-plane vibrational modes of graphite, E1u and E2g(2), are derived from graphene E2g mode, the shifts of which under compression are all considered as results of in-plane bond shortening. Values of Gruneisen parameter have been reported to quantify such relation. However, the reason why the shift rates of these three modes with pressure differ is unclear. In this work, we introduce a new parameter to quantify the contribution of out-of-plane strain to the in-plane vibrational frequencies, suggesting that the compression of \pi-electrons plays a non-negligible part in both graphite and graphene under high pressure., Comment: 8 pages, 5 figures, 1 table

The study of the G-mode pressure coefficients of carbon nanotubes, reflecting the stiff sp2 bond pressure dependence, is essential to the understanding of their extraordinary mechanical properties as well as fundamental mechanics. However, it is hindered by the availability of carbon nanotubes samples only as bundles or isolated with surfactants. Octadecylamine functionalized carbon nanotubes are mostly of a single diameter and can be stably dispersed in 1, 2-dichloroethane and chloroform without surfactants. Here we perform high pressure Raman spectroscopy on these tubes and obtain their experimental G-mode pressure coefficients for individual tubes and bundles. The G-plus pressure coefficient for bundles is only about half of that for individual tubes in 1, 2-dichloroethane and is about two-thirds in chloroform. The G-minus pressure coefficient for bundles is about one-third of G-plus in 1, 2-dichloroethane and about the same in chloroform. These results for the first time provide unambiguous experimental evidence of the significant effect of bundling on carbon nanotubes' G-mode pressure coefficients, identifying it as one of the major reasons for the lack of consensus on what the values should be in the literature., Comment: 22 pages, 1 table, 8 figures

Slow flow of the viscous liquid is a thought-provoking experiment that challenges students, academics and public to think about some fundamental questions in modern science. In the Queensland demonstration, the world-longest running experiment earning the Ig Nobel prize, one drop of pitch takes about 10 years to fall, leading to problems of actually observing the drops. Here, we describe our recent demonstration of slowly-flowing bitumen where appreciable flow is observed on the time scale of months. The experiment is free from dissipative heating effects and has the potential to improve the accuracy of measurement. Bitumen viscosity was calculated by undergraduate students during the summer project. The worldwide access to the running experiment is provided by webcams uploading the images to a dedicated website, enhancing student education experience and promotion of science. This demonstration serves as an attractive student education exercise and stimulates the discussion of fundamental concepts and hotly debated ideas in modern physics research: difference between solids and liquids, the nature of liquid-glass transition, emergence of long time scales in a physical process, and the conflict between human intuition and physical reality.

Studies of the mechanical properties of single-walled carbon nanotubes are hindered by the availability only of ensembles of tubes with a range of diameters. Tunable Raman excitation spectroscopy picks out identifiable tubes. Under high pressure, the radial breathing mode shows a strong environmental effect shown here to be largely independent of the nature of the environment . For the G-mode, the pressure coefficient varies with diameter consistent with the thick-wall tube model. However, results show an unexpectedly strong environmental effect on the pressure coefficients. Reappraisal of data for graphene and graphite gives the G-mode Grueuneisen parameter gamma = 1.34 and the shear deformation parameter beta = 1.34., Comment: Submitted to Physical Review B

Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added - water, hexane, sulphuric acid - and when the nanotubes are unbundled in water with surfactant and sonication. The shift is about 2 - 3 cm-1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously., Comment: 6 pages, 3 Figures, Proceedings of EHPRG 2011 (Paris)

Small-scale mechanical behaviour shows significant departures from classical elastic-plastic theory. In a remarkable number of instances, the strength of a material appears to scale as the reciprocal square root of the smallest length scale. There are several recent experimental and modeling results in the literature that show an interaction between dimensional (extrinsic) size and microstructural (intrinsic) size effects. In this paper, we present a mechanical model that naturally produces the inverse square root strengthening and derive an expression for the effective length when both the extrinsic and intrinsic size effects are significant. The theory fits well to data from a wide range of deformation geometries and includes the interaction between the microstructural and dimensional size effects. Furthermore, this approach is able to predict the size effect under uniform deformation without strain gradient.

The mechanism of the shift, broadening and quenching of the ammonia inversion frequency with gas pressure has been a problem of lively interest for over seventy years. A simple quantum model of the ammonia molecule perturbed by collisions with ideal gas molecules displays the essential features of the experimental data for NH3 and for ND3. The model does not display the behaviour expected from theories of quantum localisation such as quantum state diffusion and decoherence. On the other hand, models of perturbed classical oscillators do display similar behaviour to our model. The quenching of the ammonia inversion transition cannot therefore be interpreted as spatial localisation of the wavefunction., Comment: pdf format, 10 pages, 4 figures

Quantum state diffusion shows how stochastic interaction with the environment may cause localisation of the wave-function, and thereby demonstrates that quantum mechanics need not invoke a separate axiom of measurement to explain the emergence of the classical world. It has not been clear whether quantum state diffusion requires some new physics. We set up an explicit numerical calculation of the evolution of the wave-function of a two-state system under interaction using only the physics explicitly contained in quantum mechanics without an axiom of measurement. The wave-function does indeed localise, as proposed by quantum state diffusion, on eigenstates of the perturbation. The mechanism appears to be the superposition of histories evolving under different Hamiltonians., Comment: 11 pages, 1 figure