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Graphene is a powerful membrane prototype for both applications and fundamental research. Rheological phenomena including indentation, twisting, and wrinkling in deposited and suspended graphene are actively investigated to unravel the mechanical laws at the nanoscale. Most studies focused on isotropic set-ups, while realistic graphene membranes are often subject to strongly anisotropic constraints, with important consequences for the rheology, strain, indentation, and friction in engineering conditions.

The cyclic sudden switching of a magnetic impurity from Kondo to a non-Kondo state and back was recently shown to involve an important dissipation of the order of several $k_BT_K$ per cycle. The possibility to reveal this and other electronic processes through nanomechanical dissipation by e.g., ultrasensitive Atomic Force Microscope (AFM) tools currently represents an unusual and interesting form of spectroscopy. Here we explore the dependence on the switching time of the expected dissipation, a quantity whose magnitude is physically expected to drop from maximum to zero between sudden and slow switching, respectively. By applying a recently established matrix-product-state based time-dependent variational algorithm to the magnetic field-induced Kondo switching in an Anderson model of the magnetic impurity, we find that dissipation requires switching within the Kondo time scale $\hbar(k_B T_K)^{-1}$ or faster. While such a fast switching seems problematic for current AFM setups, the challenge is open for future means to detect this dissipation by time-dependent magnetic fields, electrostatic impurity level shift, or hybridization switching.

The understanding of friction at nano-scales, ruled by the regular arrangement of atoms, is surprisingly incomplete. Here we provide a unified understanding by studying the interlocking potential energy of two infinite contacting surfaces with arbitrary lattice symmetries, and extending it to finite contacts. We categorize, based purely on geometrical features, all possible contacts into three different types: a structurally lubric contact where the monolayer can move isotropically without friction, a corrugated and strongly interlocked contact, and a newly discovered directionally structurally lubric contact where the layer can move frictionlessly along one specific direction and retains finite friction along all other directions. This novel category is energetically stable against rotational perturbations and provides extreme friction anisotropy. The finite-size analysis shows that our categorization applies to a wide range of technologically relevant materials in contact, from adsorbates on crystal surfaces to layered two-dimensional materials and colloidal monolayers. published

The peeling of an immobile adsorbed membrane is a well known problem in engineering and macroscopic tribology. In the classic setup, picking up at one extreme and pulling off results in a peeling force that is a decreasing function of the pickup angle. As one end is lifted, the detachment front retracts to meet the immobile tail. At the nanoscale, interesting situations arise with the peeling of graphene nanoribbons (GNRs) on gold, as realized, e.g., by atomic force microscopy. The nanosized system shows a constant-force steady peeling regime, where the tip lifting h produces no retraction of the ribbon detachment point, and just an advancement h of the free tail end. This is opposite to the classic case, where the detachment point retracts and the tail end stands still. Here we characterise, by analytical modeling and numerical simulations, a third, experimentally relevant, setup where the nanoribbon, albeit structurally lubric, does not have a freely moving tail end, which is instead elastically tethered. Surprisingly, novel nontrivial scaling exponents appear that regulate the peeling evolution. As the detachment front retracts and the tethered tail is stretched, power laws of h characterize the shrinking of the adhered length the growth of peeling force and the peeling angle. These exponents precede the final total detachment as a critical point, where the entire ribbon eventually hangs suspended between the tip and tethering spring. These analytical predictions are confirmed by realistic MD simulations, retaining the full atomistic description, also confirming their survival at finite experimental temperatures.

Nanotubes show great promise for miniaturizing advanced technologies. Their exceptional physical properties are intimately related to their morphological and crystal structure. Circumferential faceting of multiwalled nanotubes reinforces their mechanical strength and alters their tribological and electronic properties. Here, the nature of this important phenomenon is fully rationalized in terms of interlayer registry patterns. Regardless of the nanotube identity (that is, diameter, chirality, chemical composition), faceting requires the matching of the chiral angles of adjacent layers. Above a critical diameter that corresponds well with experimental results, achiral multiwalled nanotubes display evenly spaced extended axial facets whose number equals the interlayer difference in circumferential unit cells. Elongated helical facets, commonly observed in experiment, appear in nanotubes that exhibit small interlayer chiral angle mismatch. When the wall chiralities are uncorrelated, faceting is suppressed and outer layer corrugation, which is induced by the Moire superlattice, is obtained in agreement with experiments. Finally, we offer an explanation for the higher incidence of faceting in multiwalled boron nitride nanotubes with respect to their carbon-based counterparts. The phenomenon of circumferential faceting in multiwalled nanotubes of general chirality and identity is rationalized in terms of interwall registry patterns between adjacent layers of curved hexagonal lattices.

The mechanical rigidity or softness of metal-metal nanocontacts under large vibrations is important in nanoscale rheology and in technology. A puzzling shear-induced liquefaction under oscillatory strain, totally unexpected at room temperature, was suggested by recent experiments on nanosized gold junctions. Here we show theoretically that the simulated gold nanocontact structure actually remains crystalline even under large oscillatory strains. Tensile and compressive slips, respectively of “necking” and “bellying” types, do take place, but recover reversibly even during fast oscillatory cycles. We also explain why, counterintuitively, the residual stress remains tensile after both slips, driving the averaged stiffness from positive to negative, thus superficially mimicking a liquid’s. Unlike a liquid, however, the softening of the solid junction occurs by stick-slip, predicting largely frequency independent stiffness with violent noise in stress and conductance, all properties compatible with experiments. This surprising large amplitude rheology of nanojunctions and its consequences are likely to apply, with different parameters, to many other metals.

Understanding the interfacial properties between an atomic layer and its substrate is of key interest at both the fundamental and technological level. From Fermi level pinning to strain engineering and superlubricity, the interaction between a single atomic layer and its substrate governs electronic, mechanical, and chemical properties of the layer-substrate system. Here, we measure the hardly accessible interfacial transverse shear modulus of an atomic layer on a substrate. We show that this key interfacial property is critically controlled by the chemistry, order, and structure of the atomic layer-substrate interface. In particular, the experiments demonstrate that the interfacial shear modulus of epitaxial graphene on SiC increases for bilayer films compared to monolayer films, and augments when hydrogen is intercalated between graphene and SiC. The increase in shear modulus for two layers compared to one layer is explained in terms of layer-layer and layer-substrate stacking order, whereas the increase with H-intercalation is correlated with the pinning induced by the H-atoms at the interface. Importantly, we also demonstrate that this modulus is a pivotal measurable property to control and predict sliding friction in supported two-dimensional materials. Indeed, we observe an inverse relationship between friction and interfacial shear modulus, which naturally emerges from simple friction models based on a point mass driven over a periodic potential. This inverse relation originates from a decreased dissipation in presence of large shear stiffness, which reduces the energy barrier for sliding.

Understanding the interfacial properties between an atomic layer and its substrate is of key interest at both the fundamental and technological levels. From Fermi level pinning to strain engineering and superlubricity, the interaction between a single atomic layer and its substrate governs electronic, mechanical and chemical properties. Here, we measure the hardly accessible interfacial transverse shear modulus of an atomic layer on a substrate. By performing measurements on bulk graphite, and on epitaxial graphene films on SiC with different stacking orders and twisting, as well as in the presence of intercalated hydrogen, we find that the interfacial transverse shear modulus is critically controlled by the stacking order and the atomic layer-substrate interaction. Importantly, we demonstrate that this modulus is a pivotal measurable property to control and predict sliding friction in supported two-dimensional materials. The experiments demonstrate a reciprocal relationship between friction force per unit contact area and interfacial shear modulus. The same relationship emerges from simulations with simple friction models, where the atomic layer-substrate interaction controls the shear stiffness and therefore the resulting friction dissipation.

Metal atomic chains have been reported to change their electronic or magnetic properties by slight mechanical stimulus. However, the mechanical response has been veiled because of lack of information on the bond nature. Here, we clarify the bond nature in platinum (Pt) monatomic chains by our developed in-situ transmission electron microscope method. The stiffness is measured with sub N/m precision by quartz length-extension resonator. The bond stiffnesses at the middle of the chain and at the connecting to the base are estimated to be 25 and 23 N/m, respectively, which are higher than the bulk counterpart. Interestingly, the bond length of 0.25 nm is found to be elastically stretched to 0.31 nm, corresponding to 24% in strain. Such peculiar bond nature could be explained by a novel concept of "string tension". This study is a milestone that will significantly change the way we think about atomic bonds in one-dimensional substance., 20pages, 4 figures

Understanding the drift motion and dynamical locking of crystalline clusters on patterned substrates is important for the diffusion and manipulation of nano- and micro-scale objects on surfaces. In a previous work, we studied the orientational and directional locking of colloidal two-dimensional clusters with triangular structure driven across a triangular substrate lattice. Here we show with experiments and simulations that such locking features arise for clusters with arbitrary lattice structure sliding across arbitrary regular substrates. Similar to triangular-triangular contacts, orientational and directional locking are strongly correlated via the real- and reciprocal-space moir\'e patterns of the contacting surfaces. Due to the different symmetries of the surfaces in contact, however the relation between the locking orientation and the locking direction becomes more complicated compared to interfaces composed of identical lattice symmetries. We provide a generalized formalism which describes the relation between the locking orientation and locking direction with arbitrary lattice symmetries., Comment: Published in Physical Review E. 17 pages and 10 figures including supplementary information

Atomic scale friction, an indispensable element of nanotechnology, requires a direct access to, under actual growing shear stress, its successive live phases: from static pinning, to depinning and transient evolution, eventually ushering in steady state kinetic friction. Standard tip-based atomic force microscopy generally addresses the steady state, but the prior intermediate steps are much less explored. Here we present an experimental and simulation approach, where an oscillatory shear force of increasing amplitude leads to a one-shot investigation of all these successive aspects. Demonstration with controlled gold nanocontacts sliding on graphite uncovers phenomena that bridge the gap between initial depinning and large speed sliding, of potential relevance for atomic scale time and magnitude dependent rheology.

Atomic diffusion is a spontaneous process and significantly influences properties of materials, such as fracture toughness, creep-fatigue properties, thermal conductivity, thermoelectric properties, etc. Here, using extensive molecular dynamics simulations based on both ab initio and machine-learning potentials, we demonstrate that an atomic one dimensional cooperative diffusion exists in the simple cubic high-pressure finite-temperature phase of calcium in the premelting regime, where some atoms diffuse cooperatively as chains or even rings, while others remain in the solid state. This intermediate regime is triggered by anharmonicity of the system at high temperature and is stabilized by the competition between the internal energy minimization and the entropy maximization, and has close connections with the unique electronic structures of simple cubic Ca as an electride with a pseudogap. This cooperative diffusion regime explains the abnormal enhancement of the melting line of Ca under high pressure and suggests that the cooperative chain melting is a much more common high-temperature feature among metals under extreme conditions than hitherto thought. The microscopic electronic investigations of these systems combining ab initio and machine-learning data point out the direction for further understanding of other metallic systems such as the glass transition, liquid metals, etc.

Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its low-temperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice., Comment: 11 pages, 4 figures

We investigated the electronic structure of the $\mathrm{Si}(111)--7\ifmmode\times\else\texttimes\fi{}7$ surface below 20 K by scanning tunneling and photoemission spectroscopies and by density functional theory calculations. Previous experimental studies have questioned the ground state of this surface, which is expected to be metallic in a band picture because of the odd number of electrons per unit cell. Our differential conductance spectra instead show the opening of an energy gap at the Fermi level and a significant temperature dependence of the electronic properties, especially for the adatoms at the center of the unfaulted half of the unit cell. Complementary photoemission spectra with improved correction of the surface photovoltage shift corroborate the differential conductance data and demonstrate the absence of surface bands crossing the Fermi level at 17 K. These consistent experimental observations point to an insulating ground state and contradict the prediction of a metallic surface obtained by density functional theory in the generalized gradient approximation. The calculations indicate that this surface has or is near a magnetic instability, but remains metallic in the magnetic phases even including correlation effects at mean-field level. We discuss possible origins of the observed discrepancies between experiments and calculations.

The lifting, peeling and exfoliation of physisorbed ribbons (or flakes) of 2D material such as graphene off a solid surface are common and important manoeuvres in nanoscience. The feature that makes this case peculiar is the structural lubricity generally realized by stiff 2D material contacts. We model theoretically the mechanical peeling of a nanoribbon of graphene as realized by the tip-forced lifting of one of its extremes off a flat crystal surface. The evolution of shape, energy, local curvature and body advancement are ideally expected to follow a succession of regimes: (A) initial prying, (B) peeling with stretching but without sliding (stripping), (C) peeling with sliding, (D) liftoff. In the case where in addition the substrate surface corrugation is small or negligible, then (B) disappears, and we find that the (A)-(C) transition becomes universal, analytical and sharp, determined by the interplay between bending rigidity and adsorption energy. This general two-stage peeling transition is identified as a sharp crossover in published data of graphene nanoribbons pulled off an atomic-scale Au(111) substrate., 10 pages, 5 figures

We calculate the distribution probability of hexagonal six-site rings in the disordered state of a cubic or hexagonal ice lattice model with ice rules perfectly obeyed. The mean-field distribution obtained is in significant agreement with those, slightly different among them, obtained by Monte Carlo simulations of the cubic or hexagonal model. The results are discussed in connection with the equilibrium and non-equilibrium transition from disorder to ferroelectric proton order.

Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. To date, there hasn't been any practical demonstration of the transformation of multi-layer graphene into diamond-like ultra-hard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, resisting to perforation with a diamond indenter, and showing a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp2-to-sp3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than 3 to 5 layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation., Comment: Published online on Nature Nanotechnology on December 18, 2017

We identify a new material phenomenon, where minute mechanical manipulations induce pronounced global structural reconfigurations in faceted multi-walled nanotubes. This behavior has strong implications on the tribological properties of these systems and may be the key to understand the enhanced inter-wall friction recently measured for boron-nitride nanotubes with respect to their carbon counterparts. Notably, the fast rotation of helical facets in these systems upon coaxial sliding may serve as a nanoscale Archimedean screw for directional transport of physisorbed molecules.

The surprising insulating and superconducting states of narrow-band graphene twisted bilayers have been mostly discussed so far in terms of strong electron correlation, with little or no attention to phonons and electron-phonon effects. We found that, among the 33492 phonons of a fully relaxed $\theta=1.08^\circ$ twisted bilayer, there are few special, hard and nearly dispersionless modes that resemble global vibrations of the moir\'e supercell, as if it were a single, ultralarge molecule. One of them, doubly degenerate at $\Gamma$ with symmetry $A_1+B_1$, couples very strongly with the valley degrees of freedom, also doubly degenerate, realizing a so-called $\text{E}\otimes\text{e}$ Jahn-Teller (JT) coupling. The JT coupling lifts very efficiently all degeneracies which arise from the valley symmetry, and may lead, for an average atomic displacement as small as $0.5~$mA, to an insulating state at charge neutrality. This insulator possesses a non-trivial topology testified by the odd winding of the Wilson loop. In addition, freezing the same phonon at a zone boundary point brings about insulating states at most integer occupancies of the four ultra-flat electronic bands. Following that line, we further study the properties of the superconducting state that might be stabilized by these modes. Since the JT coupling modulates the hopping between AB and BA stacked regions, pairing occurs in the spin-singlet Cooper channel at the inter-(AB-BA) scale, which may condense a superconducting order parameter in the extended $s$-wave and/or $d\pm id$-wave symmetry., Comment: Revised version published in PRX (Open access)

The contact strength, adhesion and friction, between graphene and an incommensurate crystalline substrate such as {\it h}-BN depends on their relative alignment angle $\theta$. The well established Novaco-McTague (NM) theory predicts for a monolayer graphene on a hard bulk {\it h}-BN crystal face a small spontaneous misalignment, here $\theta_{NM}$\,$\simeq$\,0.45 degrees which if realized would be relevant to a host of electronic properties besides the mechanical ones. Because experimental equilibrium is hard to achieve, we inquire theoretically about alignment or misalignment by simulations based on dependable state-of-the-art interatomic force fields. Surprisingly at first, we find compelling evidence for $\theta = 0$, i.e., full energy-driven alignment in the equilibrium state of graphene on {\it h}-BN. Two factors drive this deviation from NM theory. First, graphene is not flat, developing on {\it h}-BN a long-wavelength out-of-plane corrugation. Second, {\it h}-BN is not hard, releasing its contact stress by planar contractions/expansions that accompany the interface moir\'e structure. Repeated simulations by artificially forcing graphene to keep flat, and {\it h}-BN to keep rigid, indeed yield an equilibrium misalignment similar to $\theta_{NM}$ as expected. Subsequent sliding simulations show that friction of graphene on {\it h}-BN, small and essentially independent of misalignments in the artificial frozen state, strongly increases in the more realistic corrugated, strain-modulated, aligned state.

Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained.

Metal-surface physisorbed graphene nanoribbons (GNRs) constitute mobile nanocontacts whose interest is simultaneously mechanical, electronic, and tribological. Previous work showed that GNRs adsorbed on Au(111) generally slide smoothly and superlubrically owing to the incommensurability of their structures. We address here the nanomechanics of detachment, as realized when one end is picked up and lifted by an AFM cantilever. AFM nanomanipulations and molecular-dynamics (MD) simulations identify two successive regimes, characterized by (i) a progressively increasing local bending, accompanied by the smooth sliding of the adhered part, followed by (ii) a stick-slip dynamics involving sudden bending relaxation associated with intermittent jumps of the remaining adhered GNR segment and tail end. AFM measurements of the vertical force exhibit oscillations which, compared with MD simulations, can be associated with the successive detachment of individual GNR unit cells of length 0.42 nm. Extra modulations within one single period are caused by steplike advancements of the still-physisorbed part of the GNR. The sliding of the incommensurate moiré pattern that accompanies the GNR lifting generally yields an additional long-period oscillation: while almost undetectable when the GNR is aligned in the standard "R30" orientation on Au(111), we predict that such feature should become prominent in the alternative rotated "R0" orientation on the same surface, or on a different surface, such as perhaps Ag(111).

We discuss and qualify the connection between two separate phenomena in the physics of nanoscale friction, general in nature and relevant to experiments. The first is thermally assisted lubricity (TAL), i.e., the low-velocity regime where a nanosized dry slider exhibits a viscouslike friction despite corrugations that would otherwise imply hard stick--slip friction. The second is the occurrence of negative dissipated work (NDW) events in sampling the work probability distribution. The abundance, or scarcity due to insufficient sampling, of these NDW events implies experimental fulfillment or violation of the celebrated Jarzynski equality (JE) of nonequilibrium statistical mechanics. We show, both analytically and in simulations of the one-dimensional point slider Prandtl-Tomlinson model, that a general crossover can be individuated as the total frictional work per cycle crosses ${k}_{B}T$. Below such crossover, the TAL regime holds, the dissipation is essentially linear, and the numerical validation for the JE is feasible (i.e., does not require an exponentially large sampling size). Above it, the dissipation profile departs from linearity and gains its hard stick--slip features, and the mandatory sampling for the JE becomes exponentially large. In addition, we derive a parameter-free formula expressing the linear velocity coefficient of viscous friction, correcting previous empirically parameterized expressions. With due caution, the connection between friction and work tails can be extended beyond a single degree of freedom to more complex sliders, thus inviting realistic crosscheck experiments. Of importance for experimental nanofriction will be the search for NDW tails in the sliding behavior of trapped cold ions, and alternatively checking for TAL in the sliding pattern of dragged colloid monolayers as well as in forced protein unwinding.

Friction and its science hold more than a record. One is longevity: from its very origins, mankind has had to reckon with it. Still today, reducing (or increasing) sliding friction remain technological and practical objectives of enormous importance — one can read for example that no less than 5% of all energy produced daily degrades into wasted frictional work. Another record is that despite the involvement of great scientist like Leonardo, who already five centuries ago gave friction its first scientific bases, there is still today no proper theoretical formulation of friction. Theorists like us mostly limit themselves to what P.W.Anderson jokingly defined in a different context “the indignity of numerical simulations”. However, progress in science does not take place because it is necessary, but because it is possible. In the last decades, new mesoscopic and nanoscopic experimental techniques opened new windows on frictional phenomena at the atomic and molecular level. Jump-started by the necessity and by the challenge to understand some of that data, theory and simulation progress is moving on along some lines which I will briefly describe.

Recent model simulations discovered unexpected non-monotonic features in the wear-free dry phononic friction as a function of the sliding speed. Here we demonstrate that a rather straight- forward application of linear-response theory, appropriate in a regime of weak slider-substrate in- teraction, predicts frictional one-phonon singularities which imply a non-trivial dependence of the dynamical friction force on the slider speed and/or coupling to the substrate. The explicit formula which we derive reproduces very accurately the classical atomistic simulations when available. By modifying the slider-substrate interaction the analytical understanding obtained provides a practical means to tailor and control the speed dependence of friction with substantial freedom.

The possibility to achieve entirely frictionless, i.e. superlubric, sliding between solids, holds enormous potential for the operation of mechanical devices. At small length scales, where mechanical contacts are well-defined, Aubry predicted a transition from a superlubric to a pinned state when the mechanical load is increased. Evidence for this intriguing Aubry transition (AT), which should occur in one dimension (1D) and at zero temperature, was recently obtained in few-atom chains. Here, we experimentally and theoretically demonstrate the occurrence of the AT in an extended two-dimensional (2D) system at room temperature using a colloidal monolayer on an optical lattice. Unlike the continuous nature of the AT in 1D, we observe a first-order transition in 2D leading to a coexistence regime of pinned and unpinned areas. Our data demonstrate that the original concept of Aubry does not only survive in 2D but is relevant for the design of nanoscopic machines and devices at ambient temperature., Comment: 12 pages including 4 figures + 9 pages supplemental information

Dichalcogenides are known to exhibit layered solid phases, at ambient and high pressures, where 2D layers of chemically bonded formula units are held together by van der Waals forces. These materials are of great interest for solid-state sciences and technology, along with other 2D systems such as graphene and phosphorene. SiS

Thermal ripples of graphene are well understood at room temperature, but their quantum counterparts at low temperatures are still in need of a realistic quantitative description. Here we present atomistic path-integral Monte Carlo simulations of freestanding graphene, which show upon cooling a striking classical-quantum evolution of height and angular fluctuations. The crossover takes place at ever-decreasing temperatures for ever-increasing wavelengths so that a completely quantum regime is never attained. Zero-temperature quantum graphene is flatter and smoother than classical at large scales, yet rougher at short scales. The angular fluctuation distribution of the normals can be quantitatively described by coexistence of two Gaussians, one classical strongly T-dependent and one quantum about $2^{\circ}$ wide, of zero-point character. The quantum evolution of ripple-induced height and angular spread should be observable in electron diffraction in graphene and other two-dimensional materials like MoS$_2$, bilayer graphene, boron nitride, etc., 6 pages, 6 figures, paper is accompanied by supplementary material available in Ancillary files or inside the directory anc/ included in the source of this manuscript

The quantum motion of nuclei, generally ignored in the physics of sliding friction, can affect in an important manner the frictional dissipation of a light particle forced to slide in an optical lattice. The density matrix-calculated evolution of the quantum version of the basic Prandtl-Tomlinson model, describing the dragging by an external force of a point particle in a periodic potential, shows that purely classical friction predictions can be very wrong. The strongest quantum effect occurs not for weak but for strong periodic potentials, where barriers are high but energy levels in each well are discrete, and resonant Rabi or Landau-Zener tunneling to states in the nearest well can preempt classical stick-slip with nonnegligible efficiency, depending on the forcing speed. The resulting permeation of otherwise unsurmountable barriers is predicted to cause quantum lubricity, a phenomenon which we expect should be observable in the recently implemented sliding cold ion experiments. © 2018 National Academy of Sciences. All rights reserved.

Graphene nanoribbons (GNRs) physisorbed on a Au(111) surface can be picked up, lifted at one end, and made to slide by means of the tip of an atomic-force microscope. The dynamic transition from smooth sliding to multiple stick-slip regimes, the pushing/pulling force asymmetry, the presence of pinning, and its origin are real frictional processes in a nutshell, in need of a theoretical description. To this purpose, we conduct classical simulations of frictional manipulations of a 30 nm-long GNR, one end of which is pushed or pulled horizontally while held at different heights above the Au surface. These simulations allow us to clarify theoretically the emergence of stick-slip originating from the short 1D edges rather than the 2D "bulk", the role of adhesion, of lifting, and of graphene bending elasticity in determining the GNR sliding friction. The understanding obtained in this simple context is of additional value for more general cases.

We present a tight-binding calculation of a twisted bilayer graphene at magic angle $\theta\sim 1.08^\circ$, allowing for full, in- and out-of-plane, relaxation of the atomic positions. The resulting band structure displays as usual four narrow mini bands around the neutrality point, well separated from all other bands after the lattice relaxation. A thorough analysis of the mini-bands Bloch functions reveals an emergent $D_6$ symmetry, despite the lack of any manifest point group symmetry in the relaxed lattice. The Bloch functions at the $\Gamma$ point are degenerate in pairs, reflecting the so-called valley degeneracy. Moreover, each of them is invariant under C$_{3z}$, i.e., transforming like one-dimensional, in-plane symmetric irreducible representation of an "emergent" $D_6$ group. Out of plane, the lower doublet is even under C$_{2x}$, while the upper doublet is odd, which implies that at least eight Wannier orbitals, two $s$-like and two $p_z$-like for each of the two supercell sublattices AB and BA are necessary, probably not sufficient, to describe the four mini bands. This unexpected one-electron complexity is likely to play an important role in the still unexplained metal-insulator-superconductor phenomenology of this system., Comment: 9 pages, 8 figures, 1 table

The phase diagram of isotropically expanded graphene cannot be correctly predicted by ignoring either electron correlations, or mobile carbons, or the effect of applied stress, as was done so far. We calculate the ground state enthalpy (not just energy) of strained graphene by an accurate off-lattice Quantum Monte Carlo (QMC) correlated ansatz of great variational flexibility. Following undistorted semimetallic graphene (SEM) at low strain, multi-determinant Heitler-London correlations stabilize between $\simeq$8.5% and $\simeq$15% strain an insulating Kekule-like dimerized (DIM) state. Closer to a crystallized resonating-valence bond than to a Peierls state, the DIM state prevails over the competing antiferromagnetic insulating (AFI) state favored by density-functional calculations which we conduct in parallel. The DIM stressed graphene insulator, whose gap is predicted to grow in excess of 1 eV before failure near 15% strain, is topological in nature, implying under certain conditions 1D metallic interface states lying in the bulk energy gap., Comment: 10 pages, 7 figures

We discuss, based on first-principles calculations, the possibility of tuning the magnetism of oxygen vacancies at the (001) surface of strontium titanate $({\mathrm{SrTiO}}_{3})$. The magnetic moment of single and clustered vacancies stemming from $\mathrm{Ti}\ensuremath{-}\mathrm{O}$ broken bonds can be both quenched and stabilized controllably by chemical potential adjustment associated with doping the system with electrons or holes. We discuss to what extent this route to magnetization state control is robust against other external influences such as chemical doping, mechanical action, and electric field. Such control of the vacancy state and magnetization can conceivably be achieved experimentally by using local probe tips.

BaBiO$_3$ is a mixed-valence perovskite which escapes the metallic state through a Bi valence (and Bi-O bond) disproportionation or CDW distortion, resulting in a semiconductor with a gap of 0.8 eV at zero pressure. The evolution of structural and electronic properties at high pressure is, however, largely unknown. Pressure, one might have hoped, could reduce the disproportionation, making the two Bi ions equivalent and bringing the system closer to metallicity or even to superconductivity, such as is attained at ambient pressure upon metal doping. We address the high-pressure phase diagram of pristine BaBiO$_3$ by ab initio DFT calculations based on GGA and hybrid functionals in combination with crystal structure prediction methods based on evolutionary algorithms, molecular dynamics and metadynamics. The calculated phase diagram from 0 to 50 GPa indicates that pristine BaBiO$_3$ resists metallization under pressure, undergoing instead at room temperature structural phase transitions from monoclinic \textit{I2/m} to nearly tetragonal \textit{P-1} at 7 GPa, possibly to monoclinic \textit{C2/m} at 27 GPa, and to triclinic \textit{P1} at 43 GPa. Remarkably, all these phases sustain and in fact increase the inequivalence of two Bi neighboring sites and of their Bi-O bonds and, in all cases except semimetallic \textit{C2/m}, the associated insulating character. We then present high-pressure resistivity data which generally corroborate these results, and show that the insulating character persists at least up to 80 GPa, suggesting that the \textit{C2/m} phase is probably an artifact of the small computational cell., Comment: 8 pages, 10 figures

The Aubry unpinned-pinned transition in the sliding of two incommensurate lattices occurs for increasing mutual interaction strength in one dimension and is of second order at $T=0$, turning into a crossover at nonzero temperatures. Yet, real incommensurate lattices come into contact in two dimensions, at finite temperature, generally developing a mutual Novaco-McTague misalignment, conditions in which the existence of a sharp transition is not clear. Using a model inspired by colloid monolayers in an optical lattice as a test two-dimensional (2D) case, simulations show a sharp Aubry transition between an unpinned and a pinned phase as a function of corrugation. Unlike one dimension, the 2D transition is now of first order, and, importantly, remains well defined at $Tg0$. It is heavily structural, with a local rotation of moir\'e pattern domains from the nonzero initial Novaco-McTague equilibrium angle to nearly zero. In the temperature ($T$)-corrugation strength plane, the thermodynamical coexistence line between the unpinned and the pinned phases is strongly oblique, showing that the former has the largest entropy. This first-order Aubry line terminates with a novel critical point $T={T}_{c}$, marked by a susceptibility peak. The expected static sliding friction upswing between the unpinned and the pinned phase decreases and disappears upon heating from $T=0$ to $T={T}_{c}$. The experimental pursuit of this novel scenario is proposed.

The onset or demise of Kondo effect in a magnetic impurity on a metal surface can be triggered, as often observed, by the simple mechanical nudging of a tip. This mechanically-driven quantum phase transition must reflect in a corresponding mechanical dissipation peak; yet, this kind of effect has not been focused upon so far. Aiming at the simplest theoretical modeling, we initially treat the impurity as a non-interacting resonant level turned cyclically on and off, and obtain a dissipation per cycle which is proportional to the hybridization $\Gamma$, with a characteristic temperature dependent resonant peak value. A better treatment is obtained next by solving an Anderson impurity model by numerical renormalization group. Here, many body effects yield a dissipation whose peak value is now proportional to $T_K |\log T|$ so long as $T\sim T_K$, followed for $T\sim \Gamma$ by a second high temperature regime where dissipation is proportional to $\Gamma|\log T|$. The detectability of Kondo mechanical dissipation in atomic force microscopy is discussed., Comment: Revised version

The surface of a crystal made of roughly spherical molecules exposes, above its bulk rotational phase transition at T= T$_r$, a carpet of freely rotating molecules, possibly functioning as "nanobearings" in sliding friction. We explored by extensive molecular dynamics simulations the frictional and adhesion changes experienced by a sliding C$_{60}$ flake on the surface of the prototype system C$_{60}$ fullerite. At fixed flake orientation both quantities exhibit only a modest frictional drop of order 20% across the transition. However, adhesion and friction drop by a factor of $\sim$ 2 as the flake breaks its perfect angular alignment with the C$_{60}$ surface lattice suggesting an entropy-driven aligned-misaligned switch during pull-off at T$_r$. The results can be of relevance for sliding Kr islands, where very little frictional differences were observed at T$_r$, but also to the sliding of C$_{60}$ -coated tip, where a remarkable factor $\sim$ 2 drop has been reported.

Long after its discovery superconductivity in alkali fullerides A$_3$C$_{60}$ still challenges conventional wisdom. The freshest inroad in such ever-surprising physics is the behaviour under intense infrared (IR) excitation. Signatures attributable to a transient superconducting state extending up to temperatures ten times higher than the equilibrium $T_c\sim$ 20 K have been discovered in K$_3$C$_{60}$ after ultra-short pulsed IR irradiation -- an effect which still appears as remarkable as mysterious. Motivated by the observation that the phenomenon is observed in a broad pumping frequency range that coincides with the mid-infrared electronic absorption peak still of unclear origin, rather than to TO phonons as has been proposed, we advance here a radically new mechanism. First, we argue that this broad absorption peak represents a "super-exciton" involving the promotion of one electron from the $t_{1u}$ half-filled state to a higher-energy empty $t_{1g}$ state, dramatically lowered in energy by the large dipole-dipole interaction acting in conjunction with Jahn Teller effect within the enormously degenerate manifold of $\big(t_{1u}\big)^2\big(t_{1g}\big)^1$ states. Both long-lived and entropy-rich because they are triplets, the IR-induced excitons act as a sort of cooling mechanism that permits transient superconductive signals to persist up to much larger temperatures., Comment: 16 pages article and 19 pages supplementary notes

The textbook thermophoretic force which acts on a body in a fluid is proportional to the local temperature gradient. The same is expected to hold for the macroscopic drift behavior of a diffusive cluster or molecule physisorbed on a solid surface. The question we explore here is whether that is still valid on a 2D membrane such as graphene at short sheet length. By means of a nonequilibrium molecular dynamics study of a test system-a gold nanocluster adsorbed on free-standing graphene clamped between two temperatures [Formula: see text] apart-we find a phoretic force which for submicron sheet lengths is parallel to, but basically independent of, the local gradient magnitude. This identifies a thermophoretic regime that is ballistic rather than diffusive, persisting up to and beyond a 100-nanometer sheet length. Analysis shows that the phoretic force is due to the flexural phonons, whose flow is known to be ballistic and distance-independent up to relatively long mean-free paths. However, ordinary harmonic phonons should only carry crystal momentum and, while impinging on the cluster, should not be able to impress real momentum. We show that graphene and other membrane-like monolayers support a specific anharmonic connection between the flexural corrugation and longitudinal phonons whose fast escape leaves behind a 2D-projected mass density increase endowing the flexural phonons, as they move with their group velocity, with real momentum, part of which is transmitted to the adsorbate through scattering. The resulting distance-independent ballistic thermophoretic force is not unlikely to possess practical applications.

Tribology, which studies surfaces in contact and relative motion, includes friction, wear, and lubrication, straddling across different fields: mechanical engineering, materials science, chemistry, nanoscience, physics. This short review restricts to the last two disciplines, with a qualitative survey of a small number of recent progress areas in the physics of nanofriction., Comment: 17 pages, 7 figures

Old and novel layered structures are attracting increasing attention for their physical, electronic, and frictional properties. SiS$_2$, isoelectronic to SiO$_2$, CO$_2$ and CS$_2$, is a material whose phases known experimentally up to 6 GPa exhibit 1D chain-like, 2D layered and 3D tetrahedral structures. We present highly predictive $ab$ $initio$ calculations combined with evolutionary structure search and molecular dynamics simulations of the structural and electronic evolution of SiS$_2$ up to 100 GPa. A highly stable CdI$_2$-type layered structure, which is octahedrally coordinated with space group $P\bar{3}m1$ surprisingly appears between 4 and up to at least 100 GPa. The tetrahedral-octahedral switch is naturally expected upon compression, unlike the layered character realized here by edge-sharing SiS$_6$ octahedral units connecting within but not among sheets. The predicted phase is semiconducting with an indirect band gap of about 2 eV at 10 GPa, decreasing under pressure until metallization around 40 GPa. The robustness of the layered phase suggests possible recovery at ambient pressure, where calculated phonon spectra indicate dynamical stability. Even a single monolayer is found to be dynamically stable in isolation, suggesting that it could possibly be sheared or exfoliated from bulk $P\bar{3}m1$-SiS$_2$., 12 pages, 9 Figures