arXiv subjects

Erio Tosatti

Publications and source records attributed to Erio Tosatti.

At least 37 records · Page 2Linked to original sources

arXiv2022

Moiré-pattern evolution couples rotational and translational friction at crystalline interfaces

The sliding motion of objects is typically governed by their friction with the underlying surface. Compared to translational friction, however, rotational friction has received much less attention. Here, we experimentally and theoretically study the rotational depinning and orientational dynamics of two-dimensional colloidal crystalline clusters on periodically corrugated surfaces in the presence of magnetically exerted torques. We demonstrate that the traversing of locally commensurate areas of the moiré pattern through the edges of clusters, which is hindered by potential barriers during cluster rotation, controls its rotational depinning. The experimentally measured depinning thresholds as a function of cluster size strikingly collapse onto a universal theoretical curve which predicts the possibility of a superlow-static-torque state for large clusters. We further reveal a cluster-size-independent rotation-translation depinning transition when lattice-matched clusters are driven jointly by a torque and a force. Our work provides guidelines to the design of nanomechanical devices that involve rotational motions on atomic surfaces.

cond-mat.soft↗

arXiv2021

Peculiar atomic bond nature in platinum monatomic chains

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.

cond-mat.mes-hall↗

arXiv2021

Superconducting Chevrel phase PbMo$_{6}$S$_{8}$ from first principles

Chevrel ternary superconductors show an intriguing coexistence of molecular aspects, large electron-phonon and electron-electron correlations, which to some extent still impedes their quantitative understanding. We present a first principles study on the prototypical Chevrel compound PbMo$_{6}$S$_{8}$, including electronic, structural and vibrational properties at zero and high pressure. We confirm the presence of an extremely strong electron-phonon coupling, linked to the proximity to a R$\overline{3}$-P$\overline{1}$ structural phase transition, which weakens as the system, upon applied pressures, is driven away from the phase boundary. A detailed description of the superconducting state is obtained by means of fully \textit{ab initio} superconducting density functional theory (SCDFT). SCDFT accounts for the role of phase instability, electron-phonon coupling with different intra- and inter-molecular phonon modes, and without any empirical parameter, and accurately reproduces the experimental critical temperature and gap. This study provides the conclusive confirmation that Chevrel phases are phonon driven superconductors mitigated, however, by an uncommonly strong Coulomb repulsion. The latter is generated by the combined effect of repulsive Mo states at the Fermi energy and a band gap in close proximity to the Fermi level. This is crucial to rationalize why Chevrel phases, in spite of their extreme electron-phonon coupling, have critical temperatures below 15~K. In addition, we predict the evolution of the superconducting critical temperature as a function of the external pressure, showing an excellent agreement with available experimental data.

cond-mat.supr-con↗

arXiv2021

Pervasive orientational and directional locking at geometrically heterogeneous sliding interfaces

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 moiré 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.

cond-mat.soft↗

arXiv2021

Amplitude Nanofriction Spectroscopy

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.

cond-mat.mtrl-sci↗

arXiv2020

Proton strings and rings in atypical nucleation of ferroelectricity in ice

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.

cond-mat.mtrl-sci↗

arXiv2019

Modeling Nanoribbon Peeling

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.

cond-mat.mes-hall↗

arXiv2019

Lifted graphene nanoribbons on gold: from smooth sliding to multiple stick-slip regimes

Graphene nanoribbons (GNRs) physisorbed on a Au(111) surface can be picked up, lifted at one end, and made slide by means of the tip of an atomic-force microscope. The dynamical 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 for GNRs up to 30 nm in length, 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.

cond-mat.mes-hall↗

arXiv2018

Mechanical dissipation from charge and spin transitions in oxygen deficient SrTiO_3

Bodies in relative motion separated by a gap of a few nanometers can experience a tiny friction force. This non-contact dissipation can have various origins and can be successfully measured by a sensitive pendulum atomic force microscope tip oscillating laterally above the surface. Here, we report on the observation of dissipation peaks at selected voltage-dependent tip-surface distances for oxygen-deficient strontium titanate (SrTiO_3) surface at low temperatures (T = 5K). The observed dissipation peaks are attributed to tip-induced charge and spin state transitions in quantum-dot-like entities formed by single oxygen vacancies (and clusters thereof, possibly through a collective mechanism) at the SrTiO_3 surface, which in view of technological and fundamental research relevance of the material opens important avenues for further studies and applications.

cond-mat.str-el↗

arXiv2018

Thermolubricity and the Jarzynski equality

We discuss and qualify a previously unnoticed connection between two different phenomena in the physics of nanoscale friction, general in nature and also met in experiments including sliding emu- lations in optical lattices, and protein force spectroscopy. The first is thermolubricity, designating the condition in which a dry nanosized slider can at sufficiently high temperature and low velocity exhibit very small viscous friction f ~ v despite strong corrugations that would commonly imply hard mechanical stick-slip f ~ log(v). The second, apparently unrelated phenomenon present in externally forced nanosystems, is the occurrence of negative work tails ("free lunches") in the work probabilty distribution, tails whose presence is necessary to fulfil the celebrated Jarzynski equality of non-equilibrium statistical mechanics. Here we prove analytically and demonstrate numerically in the prototypical classical overdamped one-dimensional point slider (Prandtl-Tomlinson) model that the presence or absence of thermolubricity is exactly equivalent to satisfaction or violation of the Jarzynski equality. The divide between the two regimes, satisfaction of Jarzynski with ther- molubricity, and violation of both, simply coincides with the total frictional work per cycle falling below or above kT respectively. This concept can, with due caution, be extended to more complex sliders, thus inviting crosscheck experiments, such as searching for free lunches in cold ion sliding as well as in forced protein unwinding, and alternatively checking for a thermolubric regime in dragged colloid monolayers. As an important byproduct, we derive a parameter-free formula expressing the linear velocity coefficient of frictional dissipated power in the thermolubric viscous regime, correcting previous empirically parametrized expressions.

cond-mat.stat-mech↗

arXiv2018

Correlation-Driven Dimerization and Topological Gap Opening in Isotropically Strained Graphene

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.

cond-mat.mtrl-sci↗

arXiv2018

Finite-frequency dissipation in a driven Kondo model

Using both a resonant level model and the time-dependent Gutzwiller approximation, we study the power dissipation of a localized impurity hybridized with a conduction band when the hybridization is periodically switched on and off. The total dissipated energy is proportional to the Kondo temperature, with a non-trivial frequency dependence. At low frequencies it can be well approximated by the one of a single quench, and is obtainable analitically; at intermediate frequencies it undergoes oscillations; at high frequencies, after reaching its maximum, it quickly drops to zero. This frequency-dependent energy dissipation could be relevant to systems such as irradiated quantum dots, where Kondo can be switched at very high frequencies.

cond-mat.str-el↗

arXiv2018

Quantum and classical ripples in graphene

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.

cond-mat.mtrl-sci↗

arXiv2018

Analytic understanding and control of dynamical friction

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.

cond-mat.mes-hall↗

arXiv2018

Ultrahard carbon film from epitaxial two-layer graphene

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.

cond-mat.mtrl-sci↗

arXiv2017

Controlling the magnetism of oxygen surface vacancies in strontium titanate $\mathrm{(SrTiO_3\!)}$ through charging

We discuss, based on first principles calculations, the possibility to tune 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 Ti-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 like chemical doping, mechanical action and electric field. Such control of vacancy state and magnetization can conceivably be achieved experimentally by using local probe tips.

cond-mat.mes-hall↗

arXiv2017

Friction Anomalies at First-Order Transition Spinodals: 1T-TaS$_2$

Revealing phase transitions of solids through mechanical anomalies in the friction of nanotips sliding on their surfaces is an unconventional and instructive tool for continuous transitions, unexplored for first-order ones. Owing to slow nucleation, first-order structural transformations generally do not occur at the precise crossing of free energies, but hysteretically, near the spinodal temperatures where, below and above the thermodynamic transition temperature, one or the other metastable free energy branches terminates. The spinodal transformation, a collective one-shot event with no heat capacity anomaly, is easy to trigger by a weak external perturbations. Here we propose that even the gossamer mechanical action of an AFM tip may locally act as a surface trigger, narrowly preempting the spontaneous spinodal transformation, and making it observable as a nanofrictional anomaly. Confirming this expectation, the CCDW-NCCDW first-order transition of the important layer compound 1T-TaS$_2$ is shown to provide a demonstration of this effect.

cond-mat.mes-hall↗

arXiv2017

Ballistic thermophoresis of adsorbates on free-standing graphene

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 non-equilibrium molecular dynamics study of a test system -- a gold nanocluster adsorbed on free-standing graphene clamped between two temperatures $ΔT$ 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 hundred 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. Yet, 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.

cond-mat.mes-hall↗