arXiv subjects

Carlos P. Herrero

Publications and source records attributed to Carlos P. Herrero.

At least 19 recordsLinked to original sources

arXiv2026

Elastic properties of cubic silicon carbide with Si vacancies

We investigate how silicon vacancies modify the elastic response and mechanical stability of cubic 3C-SiC. Our approach employs path-integral molecular dynamics simulations, including the classical-nuclei limit, based on an efficient tight-binding Hamiltonian, whose accuracy is validated against density-functional-theory calculations. This framework enables a quantitative assessment of nuclear quantum effects arising from zero-point motion. Across a broad range of temperatures and hydrostatic pressures, spanning both tensile and compressive regimes, silicon vacancies are found to substantially renormalize the elastic constants $C_{11}$, $C_{12}$, and $C_{44}$, as well as the bulk modulus, relative to the defect-free crystal. Inclusion of nuclear quantum motion produces an additional softening of these elastic properties, particularly at low temperatures, demonstrating that quantum fluctuations make a measurable contribution to the mechanical response of defective SiC. Vacancies also affect the mechanical stability domain of 3C-SiC, lowering the maximum sustainable tensile pressure by approximately 4 GPa for a defect concentration of 0.016 per lattice site. These results reveal an interplay between point defects and quantum lattice fluctuations in determining the elastic behavior of SiC, providing microscopic insight relevant for both extreme-environment structural applications and defect-based quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-Induced Mechanical Instabilities in Cubic SiC: Structural and Electronic Properties

Silicon carbide is widely used in electronics, ceramics, and renewable energy due to its exceptional hardness and resistance. In this study, we investigate the effects of hydrostatic and uniaxial pressure (both compressive and tensile) on the structural and electronic properties of $3C$-SiC. Our analysis is based on atomistic molecular dynamics (MD) simulations using an efficient tight-binding Hamiltonian, whose accuracy is validated against density functional theory calculations. Moreover, to account for nuclear quantum effects, we employ path-integral MD simulations. Our results show significant changes in the direct electronic gap as a function of temperature and pressure, with a renormalization of about 80 meV due to zero-point motion. Under hydrostatic tensile pressure, the direct band gap $E_Γ$ vanishes at the material's mechanical stability limit (spinodal point, where the bulk modulus $B \to 0$). For uniaxial pressure, we observe instabilities (Young's modulus $Y \to 0$) at approximately 90 GPa for both tension and compression, where $E_Γ \to 0$. Additionally, we analyze the pressure dependence of the internal energy, lattice parameter, and bond length, along with their finite-temperature fluctuations, which exhibit anomalies near the instability points.

cond-mat.mtrl-sci↗

arXiv2025

Elastic properties of silicene: Spinodal instabilities

Silicene, a two-dimensional (2D) allotrope of silicon, has attracted significant interest for its electronic and mechanical properties, alongside its compatibility with various substrates. In this study, we investigate the structural and elastic characteristics of silicene using molecular dynamics simulations based on a tight-binding Hamiltonian, calibrated to align with density-functional theory calculations. We focus particularly on the material's elastic properties and mechanical stability, analyzing its behavior under extensive compressive and tensile in-plane stresses and across temperatures up to 1000 K. Key properties examined include in-plane area, Si--Si bond length, atomic mean-square displacements, elastic constants, and 2D compression modulus. Our findings reveal a notable reduction in stiffness elastic constants, Poisson's ratio, and compression modulus with increasing temperature. Additionally, we identify mechanical instabilities in the silicene structure at specific compressive and tensile biaxial stresses, signaling the material's stability limits or spinodal points. At the corresponding spinodal pressures, structural and elastic properties exhibit anomalies or divergences.

cond-mat.mtrl-sci↗

arXiv2024

Quantum nuclear motion in silicene: Assessing structural and vibrational properties through path-integral simulations

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.

cond-mat.mtrl-sci↗

arXiv2024

Water at negative pressure: Nuclear quantum effects

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.

physics.chem-ph↗

arXiv2024

Nuclear quantum effects in structural and elastic properties of cubic silicon carbide

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.

cond-mat.mtrl-sci↗

arXiv2023

Elastic properties and mechanical stability of bilayer graphene: Molecular dynamics simulations

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.

cond-mat.mtrl-sci↗

arXiv2023

Cubic silicon carbide under tensile pressure: Spinodal instability

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.

cond-mat.mtrl-sci↗

arXiv2022

Quantum effects in two-dimensional silicon carbide

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 ($κ= 1.0$ eV) and the 2D modulus of hydrostatic compression ($B_{xy}$ = 5.5 eV/Å$^2$) 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.

cond-mat.mtrl-sci↗

arXiv2022

Hydrogen dynamics on defective monolayer graphene

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 $ν= 2 \times 10^6$~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 $ν= 7 \times 10^2$~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.

physics.chem-ph↗

arXiv2021

Quantum effects in structural and elastic properties of graphite: Path-integral simulations

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.

cond-mat.mtrl-sci↗

arXiv2021

Isotopic effects in chair graphane

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.

cond-mat.mtrl-sci↗

arXiv2020

Isotopic effects in structural properties of graphene

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.

cond-mat.mtrl-sci↗

arXiv2020

Nuclear quantum effects in graphane

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.

physics.chem-ph↗

arXiv2020

Thermodynamic properties of graphene bilayers

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 $α_{xy}$ of the in-plane area is negative at low temperatures and positive for $T \gtrsim$ 800~K. The minimum $α_{xy}$ is $-6.6 \times 10^{-6}$ K$^{-1}$ at $T \approx 220$~K. Both in-plane ($χ_{xy}$) and out-of-plane ($χ_z$) 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 $χ_{xy} = 9.5 \times 10^{-2}$ Å$^2$/eV and $χ_z = 2.97 \times 10^{-2}$ 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.

cond-mat.mtrl-sci↗

arXiv2019

Nuclear quantum effects in graphene bilayers

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 $1.5 \times 10^{-2}$ Å 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.

cond-mat.mtrl-sci↗

arXiv2019

Self-avoiding walks and connective constants in clustered scale-free networks

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 $P(k) \sim k^{-γ}$ 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} $μ$ 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 $γ$ of the degree distribution: For $γ> 3$, $μ$ converges to a finite value as $N \to \infty$; for $γ= 3$, the size-dependent $μ_N$ diverges as $\ln N$, and for $γ< 3$ we have $μ_N \sim N^{(3 - γ) / 2}$.

cond-mat.dis-nn↗

arXiv2018

Thermal properties of graphene under tensile stress

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.

cond-mat.mtrl-sci↗