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This paper explores the interplay between quantum nuclear motion and anharmonicity, which causes nontrivial effects on the structural and dynamical characteristics of silicene, a two-dimensional (2D) allotrope of silicon with interesting electronic and mechanical properties. Employing path-integral molecular dynamics (PIMD) simulations, we investigate the quantum delocalization of nuclei, unraveling its impact on the behavior of silicene at the atomic scale. Our study reveals that this delocalization induces significant deviations in the structural parameters of silicene, influencing in-plane surface area, bond lengths, angles, compressibility, and overall lattice dynamics. Through extensive simulations, we delve into the temperature-dependent behavior between 25 and 1200 K, unveiling the role of quantum nuclear fluctuations in dictating thermal expansion and phonon spectra. The extent of nuclear quantum effects is assessed by comparing results of PIMD simulations using an efficient tight-binding Hamiltonian, with those obtained from classical molecular dynamics simulations. We compare the magnitude of quantum effects in this material with those in other related 2D crystalline solids, such as graphene and SiC monolayers. The observed quantum effects showcase non-negligible deviations from classical predictions, emphasizing the need for accurate quantum treatments in understanding the material's behavior at finite temperatures. At low $T$, the 2D compression modulus of silicene decreases by a 14% due to quantum nuclear motion., Comment: 14 pages, 9 figures

Quantum metrology and quantum sensing aim to use quantum properties to enhance measurement precision beyond what could be classically achieved. Here, we demonstrate how the analysis of the phase space structure of the classical limit of Kerr parametric oscillators can be used for determining control parameters values that lead to the squeezing of the uncertainty in position and the amplification of the quantum Fisher information. We also explore how quantum sensing can benefit from excited-state quantum phase transitions, even in the absence of a conventional quantum phase transition. The system that we consider models exciton-polariton condensates and superconducting circuits, making our study relevant for potential experimental applications., Comment: 5 pages, 2 figures

3D photoelectron angular distributions (PADs) are measured from an atomic target ionized by ultrafast, elliptical fields of opposite handedness. Comparing these PADs to one another and to numeric simulations, a difficult to avoid systematic error in their orientation is identified and subsequently corrected by imposing the dichroic symmetry by which they are necessarily related. We show that this correction can be directly applied to molecular targets in the same fields. This paves the way for measurement of enantiosensitive information which has yet to be accessed experimentally., Comment: 6 pages in main article, 5 in supplemental

Various condensed phases of water, spanning from the liquid state to multiple ice phases, have been systematically investigated under extreme conditions of pressure and temperature to delineate their stability boundaries. This study focuses on probing the mechanical stability of liquid water through path-integral molecular dynamics simulations, employing the q-TIP4P/F potential to model interatomic interactions in flexible water molecules. Temperature and pressure conditions ranging from 250 to 375 K and -0.3 to 1 GPa, respectively, are considered. This comprehensive approach enables a thorough exploration of nuclear quantum effects on various physical properties of water through direct comparisons with classical molecular dynamics results employing the same potential model. Key properties such as molar volume, intramolecular bond length, H--O--H angle, internal and kinetic energy are analyzed, with a specific focus on the effect of tensile stress. Particular attention is devoted to the liquid-gas spinodal pressure, representing the limit of mechanical stability for the liquid phase, at several temperatures. The quantum simulations reveal a spinodal pressure for water of -286 and -236 MPa at temperatures of 250 and 300 K, respectively. At these temperatures, the discernible shifts induced by nuclear quantum motion are quantified at 15 and 10 MPa, respectively. These findings contribute valuable insights into the interplay of quantum effects on the stability of liquid water under diverse thermodynamic conditions., Comment: 11 pages, 9 figures

Silicon carbide, a semiconducting material, has gained importance in the fields of ceramics, electronics, and renewable energy due to its remarkable hardness and resistance. In this study, we delve into the impact of nuclear quantum motion, or vibrational mode quantization, on the structural and elastic properties of 3C-SiC. This aspect, elusive in conventional {\it ab-initio} calculations, is explored through path-integral molecular dynamics (PIMD) simulations using an efficient tight-binding (TB) Hamiltonian. This investigation spans a wide range of temperatures and pressures, including tensile stress, adeptly addressing the quantization and anharmonicity inherent in solid-state vibrational modes. The accuracy of the TB model has been checked by comparison with density-functional-theory calculations at zero temperature. The magnitude of quantum effects is assessed by comparing PIMD outcomes with results obtained from classical molecular dynamics simulations. Our investigation uncovers notable reductions of 5%, 10%, and 4% in the elastic constants $C_{11}$, $C_{12}$, and $C_{44}$, respectively, attributed to atomic zero-point oscillations. Consequently, the bulk modulus and Poisson's ratio of 3C-SiC exhibit reduced values by 7% and 5% at low temperature. The persistence of these quantum effects in the material's structural and elastic attributes beyond room temperature underscores the necessity of incorporating nuclear quantum motion for an accurate description of these fundamental properties of SiC., Comment: 17 pages, 10 figures, 3 tables

Graphene has become in last decades a paradigmatic example of two-dimensional and so-called van-der-Waals layered materials, showing large anisotropy in their physical properties. Here we study the elastic properties and mechanical stability of graphene bilayers in a wide temperature range by molecular dynamics simulations. We concentrate on in-plane elastic constants and compression modulus, as well as on the atomic motion in the out-of-plane direction. Special emphasis is placed upon the influence of anharmonicity of the vibrational modes on the physical properties of bilayer graphene. We consider the excess area appearing in the presence of ripples in graphene sheets at finite temperatures. The in-plane compression modulus of bilayer graphene is found to decrease for rising temperature, and results to be higher than for monolayer graphene. We analyze the mechanical instability of the bilayer caused by an in-plane compressive stress. This defines a spinodal pressure for the metastability limit of the material, which depends on the system size. Finite-size effects are described by power laws for the out-of-plane mean-square fluctuation, compression modulus, and spinodal pressure. Further insight into the significance of our results for bilayer graphene is gained from a comparison with data for monolayer graphene and graphite., Comment: 16 pages, 13 figures

We demonstrate how the depleted pump of an optical parametric amplifier can be recycled for impulsive alignment of a molecular gas inside a hollow-core fiber and use such alignment for the broadening and frequency shift of the signal pulse at a center wavelength of $\sim 1300$nm. Our results combine non-adiabatic molecular alignment, self-phase modulation and Raman non-linearities. We demonstrate spectral shifts of up to 204 nm and a spectral broadening of more than one octave. We also report on the time delays at which broadening occurs, which do not coincide with any of the molecular rotational constants. Further, we encounter that maximum frequency shifts occur when the signal and pump have perpendicular polarization instead of parallel., Comment: 8 pages, 4 figures

Silicon carbide is a hard, semiconducting material presenting many polytypes, whose behavior under extreme conditions of pressure and temperature has attracted large interest. Here we study the mechanical properties of 3C-SiC over a wide range of pressures (compressive and tensile) by means of molecular dynamics simulations, using an effective tight-binding Hamiltonian to describe the interatomic interactions. The accuracy of this procedure has been checked by comparing results at T = 0 with those derived from ab-initio density-functional-theory calculations. This has allowed us to determine the metastability limits of this material and in particular the spinodal point (where the bulk modulus vanishes) as a function of temperature. At T = 300 K, the spinodal instability appears for a lattice parameter about 20% larger than that corresponding to ambient pressure. At this temperature, we find a spinodal pressure P_s = -43 GPa, which becomes less negative as temperature is raised (P_s = -37.9 GPa at 1500 K). These results pave the way for a deeper understanding of the behavior of crystalline semiconductors in a poorly known region of their phase diagrams., Comment: 9 pages, 7 figures

We present a high precision, self-referencing, common path XUV interferometer setup to produce pairs of spatially separated and independently controllable XUV pulses that are locked in phase and time. The spatial separation is created by introducing two equal but opposite wavefront tilts or using superpositions of orbital angular momentum. In our approach, we can independently control the relative phase/delay of the two optical beams with a resolution of 52 zs (zs = zeptoseconds). In order to explore the level of entanglement between the non-local photons, we compare three different beam modes: Bessel-like, and Gaussian with or without added orbital angular momentum. By reconstructing interference patterns one or two photons at a time we conclude that the beams are not entangled, yet each photon in the attosecond pulse train contains information about the entire spectrum. Our technique generates non-local, quantum equivalent XUV photons with a temporal jitter of 3 zs, just below the Compton unit of time of 8 zs. We argue that this new level of temporal precision will open the door for new dynamical QED tests. We also discuss the potential impact on other areas, such as imaging, measurements of non-locality, and molecular quantum tomography., Comment: 11 pages 5 figures and supplemental materials with 12 pages and 7 figures

The ability of spatial light modulators (SLMs) to modify the amplitude and phase of light has proved them invaluable to the optics and photonics community. In many applications the bit-depth of SLMs is a major limiting factor dictated by the digital processor. As a result, there is usually a compromise between refresh speed and bit-depth. Here we present a method to increase the effective bit-depth of SLMs which utilizes a linear slope as is commonly applied to deal with the zeroth order effect. This technique was tested using two interferometric transient absorption spectroscopy setups. Through the high harmonic generation in gases producing a train of attosecond pulses and harmonics from solids in the ultraviolet, two pulses are generated that interfere in the far field providing a measurement of the optical phase. An increase in the precision far beyond the limit dictated by the digital processor in the bit-depth was found., Comment: Published in Applied Optics. 10 pages, 6 figures

Two-dimensional (2D) silicon carbide is an emergent direct band-gap semiconductor, recently synthesized, with potential applications in electronic devices and optoelectronics. Here, we study nuclear quantum effects in this 2D material by means of path-integral molecular dynamics (PIMD) simulations in the temperature range from 25 to 1500~K. Interatomic interactions are modeled by a tight-binding Hamiltonian fitted to density-functional calculations. Quantum atomic delocalization combined with anharmonicity of the vibrational modes cause changes in structural and thermal properties of 2D SiC, which we quantify by comparison of PIMD results with those derived from classical molecular dynamics simulations, as well as with those given by a quantum harmonic approximation. Nuclear quantum effects are found to be appreciable in structural properties such as the layer area and interatomic distances. Moreover, we consider a {\em real} area for the SiC sheet, which takes into account bending and rippling at finite temperatures. Differences between this area and the in-plane area are discussed in the context of quantum atomic dynamics. The bending constant ($\kappa = 1.0$ eV) and the 2D modulus of hydrostatic compression ($B_{xy}$ = 5.5 eV/\AA$^2$) are clearly lower than the corresponding values for graphene. This study paves the way for a deeper understanding of the elastic and mechanical properties of 2D SiC., Comment: 14 pages, 11 figures

The hydrogen dynamics on a graphene sheet is studied in the presence of carbon vacancies. We analyze the motion of atomic H by means of molecular dynamics (MD) simulations, using a tight-binding Hamiltonian fitted to density-functional calculations. Hydrogen passivates the dangling bonds of C atoms close to a vacancy, forming C--H bonds with H located at one or the other side of the layer plane. The hydrogen dynamics has been studied from statistical analysis of MD trajectories, along with the autocorrelation function of the atomic coordinates. For a single H atom, we find an effective barrier of 0.40~eV for crossing the graphene layer, with a jump rate $\nu = 2 \times 10^6$~s$^{-1}$ at 300~K. The atomic jumps behave as stochastic events, and their number for a given temperature and time interval follows a Poisson probability distribution. For two H atoms close to a vacancy, strong correlations in the atomic dynamics are found, with a lower jump frequency $\nu = 7 \times 10^2$~s$^{-1}$ at room temperature. These results provide insight into the diffusion mechanisms of hydrogen on graphene, paving the way for a complete understanding of its motion through defective crystalline membranes., Comment: 13 pages, 11 figures

Since its inception, velocity map imaging (VMI) has been a powerful tool for measuring the 2D momentum distribution of photoelectrons generated by strong laser fields. There has been continued interest in expanding it into 3D measurements either through reconstructive or direct methods. Recently much work has been devoted to the latter of these, particularly by relating the electron time-of-flight (TOF) to the third momentum component. The technical challenge here is having timing resolution sufficient to resolve structure in the narrow (< 10 ns) electron TOF spread. Here we build upon work in the fields of VMI lens design and 3D VMI measurement by using a plano-convex thick-lens VMI in conjunction with an event-driven camera (TPX3CAM) providing TOF information for high resolution 3D electron momentum measurements. We perform simulations to show that, with the addition of a mesh electrode to the thick-lens VMI geometry, a plano-convex electrostatic field is formed which extends the detectable electron cutoff energy range while retaining high resolution. Further, the thick-lens also extends the electron TOF range which allows for better resolution of the momentum along this axis. We experimentally demonstrate these capabilities by examining above-threshold ionization in Xenon where the apparatus is shown to collect electrons of energy up to $\sim$7 eV with a TOF spread of $\sim$30 ns, both of which are improvements on previous work by factors of $\sim$1.4 and $\sim$3.75 respectively. Finally, the PCTL-VMI is equipped with a coincident ion TOF spectrometer which is shown to effectively extract unique 3D momentum distributions for different ionic species within a gas mixture. These techniques have potential to lend themselves to more advanced measurements, particularly involving systems where the electron momentum distributions possess non-trivial symmetries and require high resolution.

Ultra-fast laser pulses have made possible the study of extreme nonlinear processes. Many of these experiments however require a full characterization of the electric field, which typically requires a separate parametric optical process. As the central wavelength range of new sources continues to increase so too does the need for nonlinear crystals suited for characterizing these differing wavelengths. Here we report on the use of polycrystalline Zinc Selenide as a universal nonlinear crystal in the frequency resolved optical gating characterization technique from the near to long-wavelength infrared, capable of phase matching second-harmonic generation and sum-frequency generation over a decade of bandwidth, as well as ultra-broadband pulses, all of which being crystal orientation and input polarization independent. With the majority of ultra-fast laser sources being in this span of wavelengths, this work demonstrates a greatly simplified approach towards ultra-fast pulse characterization in the mid to long-wavelength infrared., Comment: 5 pages, 4 figures

The study of nanomaterials is an active area of research for technological applications as well as fundamental science. A common method for studying properties of isolated nanoparticles is by an in-vacuum particle beam produced via an aerodynamic lens. Despite being common practice, characterization of such beams has proven difficult as light scattering detection techniques fail for particles with sizes beyond the diffraction limit. Here we present a new technique for characterizing such nanoparticle beams using strong field ionization. By focusing an ultrafast, mJ-level laser into the particle beam, a nanoparticle within the laser focus is ionized and easily detected by its ejected electrons. This method gives direct access to the nanoparticle density at the location of the focus and, by scanning the focus through the transverse and longitudinal profiles of the particle beam, the 3-dimensional particle density distribution can be attained. Further, we show that strong field ionization is effective in detecting spherical nanoparticles as small as 10 nm in diameter. Additionally, this technique is an effective tool in optimizing the particle beam for specific applications. As an example we show that the particle beam density and width can be manipulated by restricting the gas flow into the aerodynamic lens.

Graphite, as a well-known carbon-based solid, is a paradigmatic example of the so-called van der Waals layered materials, which display a large anisotropy in their physical properties. Here we study quantum effects in structural and elastic properties of graphite by using path-integral molecular dynamics simulations in the temperature range from 50 to 1500~K. This method takes into account quantization and anharmonicity of vibrational modes in the material. Our results are compared with those found by using classical molecular dynamics simulations. We analyze the volume and in-plane area as functions of temperature and external stress. The quantum motion is essential to correctly describe the in-plane and out-of-plane thermal expansion. Quantum effects cause also changes in the elastic properties of graphite with respect to a classical model. At low temperature we find an appreciable decrease in the linear elastic constants, mainly in $C_{12}$ and $C_{44}$. Quantum corrections in stiffness constants can be in some cases even larger than 20\%. The bulk modulus and Poisson's ratio are reduced in a 4\% and 19\%, respectively, due to zero-point motion of the C atoms. These quantum effects in structural and elastic properties of graphite are nonnegligible up to temperatures higher than 300~K., Comment: 15 pages, 10 figures, 5 tables

The strong-field control of plasmonic nanosystems opens up new perspectives for nonlinear plasmonic spectroscopy and petahertz electronics. Questions, however, remain regarding the nature of nonlinear light-matter interactions at sub-wavelength spatial and ultrafast temporal scales. Addressing this challenge, we investigated the strong-field control of the plasmonic response of Au nanoshells with a SiO$_2$ core to an intense laser pulse. We show that the photoelectron energy spectrum from these core-shell nanoparticles displays a striking transition between the weak and strong-field regime. This observed transition agrees with the prediction of our modified Mie-theory simulation that incorporates the nonlinear dielectric nanoshell response. The demonstrated intensity-dependent optical control of the plasmonic response in prototypical core-shell nanoparticles paves the way towards ultrafast switching and opto-electronic signal modulation with more complex nanostructures.

Graphane is a layered material consisting of a sheet of hydrogenated graphene, with a C:H ratio of 1:1. We study isotopic effects in the properties of chair graphane, where H atoms alternate in a chairlike arrangement on both sides of the carbon layer. We use path-integral molecular dynamics simulations, which allows one to analyze the influence of nuclear quantum effects on equilibrium variables of materials. Finite-temperature properties of graphane are studied in the range 50--1500~K as functions of the isotopic mass of the constituent atoms, using an efficient tight-bonding potential. Results are presented for kinetic and internal energy, atomic mean-square displacements, fluctuations in the C--H bond direction, plus interatomic distances and layer area. At low temperature, substituting $^{13}$C for $^{12}$C gives a fractional change of $-2.6 \times 10^{-4}$ in C--C distance and $-3.9 \times 10^{-4}$ in the graphane layer area. Replacing $^2$H for $^1$H causes a larger fractional change in the C--H bond of $-5.7 \times 10^{-3}$. The isotopic effect in C--C bond distance increases (decreases) by applying a tensile (compressive) in-plane stress. These results are interpreted in terms of a quasiharmonic approximation for the vibrational modes. Similarities and differences with isotopic effects in graphene are discussed., Comment: 17 pages, 11 figures, 2 tables

We argue that it is beneficial to tightly couple the widely-used Robot Operating System with Conda, a cross-platform, language-agnostic package manager, and Jupyter, a web-based interactive computational environment affording scientific computing. We provide new ROS packages for Conda, enabling the installation of ROS alongside data-science and machine-learning packages with ease. Multiple ROS versions (currently ROS1 Melodic and Noetic, as well as ROS2 Foxy and Galactic) can run simultaneously on one machine, with pre-compiled binaries available for Linux, Windows and OSX, and the ARM architecture (e.g. the Raspberry Pi and the new Apple Silicon). To deal with the large size of the ROS ecosystem, we significantly improved the speed of the Conda solver and build system by rewriting the crucial parts in C++. We further contribute a collection of JupyterLab extensions for ROS, including plugins for live plotting, debugging and robot control, as well as tight integration with Zethus, an RViz like visualization tool. Taken together, RoboStack combines the best of the data-science and robotics worlds to help researchers and developers to build custom solutions for their academic and industrial projects., Comment: IEEE Robotics & Automation Magazine

We report on an asymmetric high energy dual optical parametric amplifier (OPA) which is capable of having either the idlers, signals, or depleted pumps, relatively phase locked at commensurate or incommensurate wavelengths. Idlers and signals can be locked on the order of 200 mrad rms or better, corresponding to a 212 as jitter at $\lambda$=2$\mu$m. The high energy arm of the OPA outputs a combined 3.5 mJ of signal and idler, while the low energy arm outputs 1.5 mJ, with the entire system being pumped with a 1 kHz, 18 mJ Ti:Sapphire laser. Both arms are independently tunable from 1080 nm-2600 nm. The combination of relative phase locking, high output power and peak intensity, and large tunability makes our OPA an ideal tool for use in difference frequency generation (DFG) in the strong pump regime, and for high peak field waveform synthesis in the near-infrared. To demonstrate this ability we generate terahertz radiation through two color waveform synthesis in air plasma and show the influence of the relative phase on the generated terahertz intensity. The ability to phase lock multiple incommensurate wavelengths at high energies opens the door to a multitude of possibilities of strong pump DFG and waveform synthesis., Comment: 10 Pages, 5 figures, Submitted to Optics Express (2021)

A promising alternative to Gaussian beams for use in strong field science is Bessel-Gauss (BG or Bessel-like) laser beams as they are easily produced with readily available optics and provide more flexibility of the spot size and working distances. Here we use BG beams produced with a lens-axicon optical system for higher order harmonic generation (HHG) in a thin gas jet. The finite size of the interaction region allows for scans of the HHG yield along the propagation axis. Further, by measuring the ionization yield in unison with the extreme ultraviolet (XUV) we are able to distinguish regions of maximum ionization from regions of optimum XUV generation. This distinction is of great importance for BG fields as the generation of BG beams with axicons often leads to oscillations of the on-axis intensity, which can be exploited for extended phase matching conditions. We observed such oscillations in the ionization and XUV flux along the propagation axis for the first time. As it is the case for Gaussian modes, the harmonic yield is not maximum at the point of highest ionization. Finally, despite Bessel beams having a hole in the center in the far field, the XUV beam is well collimated making BG modes a great alternative when spatial filtering of the fundamental is desired., Comment: Submitted to JOSA B, 8 pages, 4 figures

The non-linear response of dielectrics to intense, ultrashort electric fields has been a sustained topic of interest for decades with one of its most important applications being femtosecond laser micro/nano-machining. More recently, renewed interests in strong field physics of solids were raised with the advent of mid-infrared femtosecond laser pulses, such as high-order harmonic generation, optical-field-induced currents, etc. All these processes are underpinned by photoionization (PI), namely the electron transfer from the valence to the conduction bands, on a time scale too short for phononic motion to be of relevance. Here, in hexagonal boron nitride, we reveal that the bandgap can be finely manipulated by femtosecond laser pulses as a function of field polarization direction with respect to the lattice, in addition to the field's intensity. It is the modification of bandgap that enables the ultrafast PI processes to take place in dielectrics. We further demonstrate the validity of the Keldysh theory in describing PI in dielectrics in the few TW/cm2 regime., Comment: 13 pages, 4 figures

Isotopic effects are relevant to understand several properties of solids, and have been thoroughly analyzed along the years. These effects may depend on the dimensionality of the considered solid. Here we assess their magnitude for structural properties of graphene, a paradigmatic two-dimensional material. We use path-integral molecular dynamics simulations, a well-suited technique to quantify the influence of nuclear quantum effects on equilibrium variables, especially in cases where anharmonic effects are important. Emphasis is put on interatomic distances and mean-square displacements, as well as on the in-plane area of the graphene layer. At low temperature, the relative difference in C--C distance for $^{13}$C and $^{14}$C, with respect to $^{12}$C, is found to be $-2.5$ and $-4.6 \times 10^{-4}$, respectively, larger than in three-dimensional carbon-based materials such as diamond. For the in-plane area, the relative changes amount to $-3.9$ and $-6.9 \times 10^{-4}$. The magnitude of anharmonicity in the lattice vibrations is estimated by comparing the internal energy and atomic vibrational amplitudes with those derived from a harmonic approximation., Comment: 12 pages, 8 figures

Graphane is a quasi-two-dimensional material consisting of a single layer of fully hydrogenated graphene, with a C:H ratio of 1. We study nuclear quantum effects in the so-called chair-graphane by using path-integral molecular dynamics (PIMD) simulations. The interatomic interactions are modeled by a tight-binding potential model fitted to density-functional calculations. Finite-temperature properties are studied in the range from 50 to 1500~K. To assess the magnitude of nuclear quantum effects in the properties of graphane, classical molecular dynamics simulations have been also performed. These quantum effects are significant in structural properties such as interatomic distances and layer area at finite temperatures. The in-plane compressibility of graphane is found to be about twice larger than that of graphene, and at low temperature it is 9\% higher than the classical calculation. The thermal expansion coefficient resulting from PIMD simulations vanishes in the zero-temperature limit, in agreement with the third law of Thermodynamics., Comment: 14 pages, 11 figures

Thermodynamic properties of graphene bilayers are studied by path-integral molecular dynamics (PIMD) simulations, considering quantization of vibrational modes and anharmonic effects. Bilayer graphene has been studied at temperatures between 12 and 1500~K for zero external stress, using the LCBOPII effective potential. We concentrate on the thermal expansion, in-plane and out-of-plane compressibility, and specific heat. Additional insight into the meaning of our results for bilayer graphene is obtained from a comparison with data obtained from PIMD simulations for monolayer graphene and graphite. They are also analyzed in view of experimental data for graphite. Zero-point and thermal effects on the in-plane and "real" area of bilayer graphene are studied. The thermal expansion coefficient $\alpha_{xy}$ of the in-plane area is negative at low temperatures and positive for $T \gtrsim$ 800~K. The minimum $\alpha_{xy}$ is $-6.6 \times 10^{-6}$ K$^{-1}$ at $T \approx 220$~K. Both in-plane ($\chi_{xy}$) and out-of-plane ($\chi_z$) compressibilities of graphene bilayers are found to increase for rising temperature, and turn out to be lower than that corresponding to monolayer graphene and higher than those found for graphite. At 300 K, we find for the bilayer $\chi_{xy} = 9.5 \times 10^{-2}$ \AA$^2$/eV and $\chi_z = 2.97 \times 10^{-2}$ GPa$^{-1}$. Results for the specific heat obtained from the simulations are compared with those given by a harmonic approximation for the vibrational modes. This approach is noticeably accurate at temperatures lower than 200~K., Comment: 14 pages, 9 figures

We present the results of a systematic study of photoelectron emission from gasphase dielectric nanoparticles (SiO2) irradiated by intense 25 fs, 780 nm linearly polarized laser pulses as a function of particle size (20 nm to 750 nm in diameter) and laser intensity. We also introduce an experimental technique to reduce the effects of focal volume averaging. The highest photoelectron energies show a strong size dependence, increasing by a factor of six over the range of particles sizes studied at a fixed intensity. For smaller particle sizes (up to 200 nm), our findings agree well with earlier results obtained with few-cycle, ~4 fs pulses. For large nanoparticles, which exhibit stronger near-field localization due to field-propagation effects, we observe the emission of much more energetic electrons, reaching energies up to ~200 times the ponderomotive energy. This strong deviation in maximum photoelectron energy is attributed to the increase in ionization and charge interaction for many-cycle pulses at similar intensities., Comment: 14 pages, 5 figures

Graphene bilayers display peculiar electronic and mechanical characteristics associated to their two-dimensional character and relative disposition of the sheets. Here we study nuclear quantum effects in graphene bilayers by using path-integral molecular dynamics simulations, which allow us to consider quantization of vibrational modes and study the effect of anharmonicity on physical variables. Finite-temperature properties are analyzed in the range from 12 to 2000~K. Our results for graphene bilayers are compared with those found for graphene monolayers and graphite. Nuclear quantum effects turn out to be appreciable in the layer area and interlayer distance at finite temperatures. Differences in the behavior of in-plane and real areas of the graphene sheets are discussed. The interlayer spacing has a zero-point expansion of $1.5 \times 10^{-2}$ \AA\ with respect to the classical minimum. The compressibility of graphene bilayers in the out-of-plane direction is found to be similar to that of graphite at low temperature, and increases faster as temperature is raised. The low-temperature compressibility increases by a 6\% due to zero-point motion. Especial emphasis is laid upon atomic vibrations in the out-of-plane direction. Quantum effects are present in these vibrational modes, but classical thermal motion becomes dominant over quantum delocalization for large system size. The significance of anharmonicities in this atomic motion is estimated by comparing with a harmonic approximation for the vibrational modes in graphene bilayers., Comment: 15 pages, 10 figures

In this work we demonstrate the generation of two intense, ultrafast laser pulses that allow a controlled interferometric measurement of higher harmonic generation pulses with 12.8 attoseconds in resolution (half the atomic unit of time) and a precision as low as 680 zeptoseconds ($10^{-21}$ seconds). We create two replicas of a driving femtosecond pulse which share the same optical path except at the focus where they converge to two foci. An attosecond pulse train emerges from each focus through the process of HHG. The two attosecond pulse trains from each focus interfere in the far field producing a clear interference pattern in the XUV region. By controlling the relative optical phase between the two driving laser pulses we are able to actively influence the delay between the pulses and are able to perform very stable and precise pump-probe experiments. Because of the phase operation occurs across the entire spatial profile we effectively create two indistinguishable intense laser pulses or a common path interferometer for attosecond pulses. Commonality across the two beams means that they are extremely stable to environmental and mechanical fluctuations up to a Rayleigh range from the focus. In our opinion this represents an ideal source for homodyne and heterodyne spectroscopic measurements with sub-attosecond precision., Comment: 9 pages, 5 figures

Various types of walks on complex networks have been used in recent years to model search and navigation in several kinds of systems, with particular emphasis on random walks. This gives valuable information on network properties, but self-avoiding walks (SAWs) may be more suitable than unrestricted random walks to study long-distance characteristics of complex systems. Here we study SAWs in clustered scale-free networks, characterized by a degree distribution of the form $P(k) \sim k^{-\gamma}$ for large $k$. Clustering is introduced in these networks by inserting three-node loops (triangles). The long-distance behavior of SAWs gives us information on asymptotic characteristics of such networks. The number of self-avoiding walks, $a_n$, has been obtained by direct enumeration, allowing us to determine the {\em connective constant} $\mu$ of these networks as the large-$n$ limit of the ratio $a_n / a_{n-1}$. An analytical approach is presented to account for the results derived from walk enumeration, and both methods give results agreeing with each other. In general, the average number of SAWs $a_n$ is larger for clustered networks than for unclustered ones with the same degree distribution. The asymptotic limit of the connective constant for large system size $N$ depends on the exponent $\gamma$ of the degree distribution: For $\gamma > 3$, $\mu$ converges to a finite value as $N \to \infty$; for $\gamma = 3$, the size-dependent $\mu_N$ diverges as $\ln N$, and for $\gamma < 3$ we have $\mu_N \sim N^{(3 - \gamma) / 2}$., Comment: 13 pages, 9 figures

Thermal properties of graphene display peculiar characteristics associated to the two-dimensional nature of this crystalline membrane. These properties can be changed and tuned in the presence of applied stresses, both tensile and compressive. Here we study graphene monolayers under tensile stress by using path-integral molecular dynamics (PIMD) simulations, which allows one to take into account quantization of vibrational modes and analyze the effect of anharmonicity on physical observables. The influence of the elastic energy due to strain in the crystalline membrane is studied for increasing tensile stress and for rising temperature (thermal expansion). We analyze the internal energy, enthalpy, and specific heat of graphene, and compare the results obtained from PIMD simulations with those given by a harmonic approximation for the vibrational modes. This approximation turns out to be precise at low temperatures, and deteriorates as temperature and pressure are increased. At low temperature the specific heat changes as $c_p \sim T$ for stress-free graphene, and evolves to a dependence $c_p \sim T^2$ as the tensile stress is increased. Structural and thermodynamic properties display nonnegligible quantum effects, even at temperatures higher than 300~K. Moreover, differences in the behavior of the in-plane and real areas of graphene are discussed, along with their associated properties. These differences show up clearly in the corresponding compressibility and thermal expansion coefficient., Comment: 15 pages, 9 figures. arXiv admin note: text overlap with arXiv:1709.05148

Finite-temperature properties of graphene monolayers under tensile stress have been studied by path-integral molecular dynamics (PIMD) simulations. This method allows one to consider the quantization of vibrational modes in these crystalline membranes and to analyze the influence of anharmonic effects in the membrane properties. Quantum nuclear effects turn out to be appreciable in structural and thermodynamic properties of graphene at low temperature, and they can even be noticeable at room temperature. Such quantum effects become more relevant as the applied stress is increased, mainly for properties related to out-of-plane atomic vibrations. The relevance of quantum dynamics in the out-of-plane motion depends on the system size, and is enhanced by tensile stress. For applied tensile stresses, we analyze the contribution of the elastic energy to the internal energy of graphene. Results of PIMD simulations are compared with calculations based on a harmonic approximation for the vibrational modes of the graphene lattice. This approximation describes rather well the structural properties of graphene, provided that the frequencies of ZA (flexural) acoustic modes in the transverse direction include a pressure-dependent correction., Comment: 13 pages, 10 figures

Thermal properties of graphene monolayers are studied by path-integral molecular dynamics (PIMD) simulations, which take into account the quantization of vibrational modes in the crystalline membrane, and allow one to consider anharmonic effects in these properties. This system was studied at temperatures in the range from 12 to 2000~K and zero external stress, by describing the interatomic interactions through the LCBOPII effective potential. We analyze the internal energy and specific heat and compare the results derived from the simulations with those yielded by a harmonic approximation for the vibrational modes. This approximation turns out to be rather precise up to temperatures of about 400~K. At higher temperatures, we observe an influence of the elastic energy, due to the thermal expansion of the graphene sheet. Zero-point and thermal effects on the in-plane and "real" surface of graphene are discussed. The thermal expansion coefficient $\alpha$ of the real area is found to be positive at all temperatures, in contrast to the expansion coefficient $\alpha_p$ of the in-plane area, which is negative at low temperatures, and becomes positive for $T \gtrsim$ 1000~K., Comment: 13 pages, 10 figures

Path-integral molecular dynamics (PIMD) simulations have been carried out to study the influence of quantum dynamics of carbon atoms on the properties of a single graphene layer. Finite-temperature properties were analyzed in the range from 12 to 2000~K, by using the LCBOPII effective potential. To assess the magnitude of quantum effects in structural and thermodynamic properties of graphene, classical molecular dynamics simulations have been also performed. Particular emphasis has been laid on the atomic vibrations along the out-of-plane direction. Even though quantum effects are present in these vibrational modes, we show that at any finite temperature classical-like motion dominates over quantum delocalization, provided that the system size is large enough. Vibrational modes display an appreciable anharmonicity, as derived from a comparison between kinetic and potential energy of the carbon atoms. Nuclear quantum effects are found to be appreciable in the interatomic distance and layer area at finite temperatures. The thermal expansion coefficient resulting from PIMD simulations vanishes in the zero-temperature limit, in agreement with the third law of thermodynamics., Comment: 14 pages, 11 figures

Effects of pressure and temperature on structural and thermodynamic properties of ice VII have been studied by using path-integral molecular dynamics (PIMD) simulations. Temperatures between 25 and 450 K, as well as pressures up to 12 GPa were considered. Interatomic interactions were modeled by using the effective q-TIP4P/F potential for flexible water. We analyze the pressure dependence of the molar volume, bulk modulus, interatomic distances, kinetic energy, and atomic delocalization at various temperatures. Results of PIMD simulations are compared with those derived from a quasi-harmonic approximation (QHA) of vibrational modes, which helps to assess the importance of anharmonic effects, as well as the influence of the different modes on the properties of ice VII. The accuracy of the QHA for describing this high-pressure phase decreases for rising temperature, but this approximation becomes more reliable as pressure grows, since anharmonicity becomes less relevant. Comparisons with low-pressure cubic ice are presented., Comment: 15 pages, 12 figures

The Ising model in clustered scale-free networks has been studied by Monte Carlo simulations. These networks are characterized by a degree distribution of the form P(k) ~ k^(-gamma) for large k. Clustering is introduced in the networks by inserting triangles, i.e., triads of connected nodes. The transition from a ferromagnetic (FM) to a paramagnetic (PM) phase has been studied as a function of the exponent gamma and the triangle density. For gamma > 3 our results are in line with earlier simulations, and a phase transition appears at a temperature T_c(gamma) in the thermodynamic limit (system size N to infinity). For gamma <= 3, a FM-PM crossover appears at a size-dependent temperature T_co, so that the system remains in a FM state at any finite temperature in the limit N to infinity. Thus, for gamma = 3, T_co scales as ln N, whereas for gamma < 3, we find T_co ~ J N^z, where the exponent z decreases for increasing gamma. Adding motifs (triangles in our case) to the networks causes an increase in the transition (or crossover) temperature for exponent gamma > 3 (or <= 3). For gamma > 3, this increase is due to changes in the mean values < k > and < k^2 >, i.e., the transition is controlled by the degree distribution (nearest neighbor connectivities). For gamma <= 3, however, we find that clustered and unclustered networks with the same size and distribution P(k) have different crossover temperature, i.e., clustering favors FM correlations, thus increasing the temperature T_co. The effect of a degree cutoff k_cut on the asymptotic behavior of T_co is discussed., Comment: 13 pages, 12 figures

The configurational entropy of several H-disordered ice polymorphs is calculated by means of a thermodynamic integration along a path between a totally H-disordered state and one fulfilling the Bernal-Fowler ice rules. A Monte Carlo procedure based on a simple energy model is used, so that the employed thermodynamic path drives the system from high temperatures to the low-temperature limit. This method turns out to be precise enough to give reliable values for the configurational entropy of different ice phases in the thermodynamic limit (number of molecules N --> infinity). The precision of the method is checked for the ice model on a two-dimensional square lattice. Results for the configurational entropy are given for H-disordered arrangements on several polymorphs, including ices Ih, Ic, II, III, IV, V, VI, and XII. The highest and lowest entropy values correspond to ices VI and XII, respectively, with a difference of 3.3\% between them. The dependence of the entropy on the ice structures has been rationalized by comparing it with structural parameters of the various polymorphs, such as the mean ring size. A particularly good correlation has been found between the configurational entropy and the connective constant derived from self-avoiding walks on the ice networks., Comment: 12 pages, 8 figures, 3 tables

The path-integral formulation of the statistical mechanics of quantum many-body systems is described, with the purpose of introducing practicaltechniques for the simulation of solids. Monte Carlo and molecular dynamics methods for distinguishable quantum particles are presented, with particular attention to the isothermal-isobaric ensemble. Applications of these computational techniques to different types of solids are reviewed, including noble-gas solids (helium and heavier elements), group-IV materials (diamond and elemental semiconductors), and molecular solids (with emphasis on hydrogen and ice). Structural, vibrational, and thermodynamic properties of these materials are discussed. Applications also include point defects in solids (structure and diffusion), as well as nuclear quantum effects in solid surfaces and adsorbates. Different phenomena are discussed, as solid-to-solid and orientational phase transitions, rates of quantum processes, classical-to-quantum crossover, and various finite-temperature anharmonic effects (thermal expansion, isotopic effects, electron-phonon interactions). Nuclear quantum effects are most remarkable in the presence of light atoms, so that especial emphasis is laid on solids containing hydrogen as a constituent element or as an impurity., Comment: 39 pages, 25 figures

Topological properties of crystalline ice structures are studied by means of self-avoiding walks on their H-bond networks. The number of self-avoiding walks, C_n, for eight ice polymorphs has been obtained by direct enumeration up to walk length n = 27. This has allowed us to determine the `connective constant' or effective coordination number `mu' of these structures as the limit of the ratio C_n/C_{n-1} for large n. This structure-dependent parameter `mu' is related with other topological characteristics of ice polymorphs, such as the mean and minimum ring size, or the topological density of network sites. A correlation between the connective constant and the configurational entropy of hydrogen-disordered ice structures is discussed., Comment: 8 pages, 6 figures, 2 tables

Topological properties of crystalline ice structures are studied by considering ring statistics, coordination sequences, and topological density of different ice phases. The coordination sequences (number of sites at topological distance k from a reference site) have been obtained by direct enumeration until at least 40 coordination spheres for different ice polymorphs. This allows us to study the asymptotic behavior of the mean number of sites in the k-th shell, M_k, for high values of k: M_k ~ a k^2, a being a structure-dependent parameter. Small departures from a strict parabolic dependence have been studied by considering first and second differences of the series {M_k} for each structure. The parameter a ranges from 2.00 for ice VI to 4.27 for ice XII, and is used to define a topological density for these solid phases of water. Correlations between such topological density and the actual volume of ice phases are discussed. Ices Ih and Ic are found to depart from the general trend in this correlation due to the large void space in their structures., Comment: 10 pages, 7 figures, 3 tables