arXiv subjects

Gerardo G. Naumis

Publications and source records attributed to Gerardo G. Naumis.

At least 55 records · Page 3Linked to original sources

arXiv2019

Kubo conductivity for anisotropic tilted Dirac semimetals and its application to 8-Pmmn borophene: The role of different frequency, temperature and scattering limits

The electronic and optical conductivities for anisotropic tilted Dirac semimetals are calculated using the Kubo formula. As in graphene, it is shown that the minimal conductivity is sensitive to the order in which the temperature, frequency and scattering limits are taken. Both intraband and interband scattering are found to be direction dependent. In the high frequency and low temperature limit, the conductivities do not depend on frequency and are weighted by the anisotropy in such a way that the geometrical mean $\sqrt{σ_{xx}σ_{yy}}$ of the conductivity is the same as in graphene. This results from the fact that in the zero temperature limit, interband transitions are not affected by the tilt in the dispersion, a result that is physically interpreted as a global tilting of the allowed transitions. Such result is verified by an independent and direct calculation of the absorption coefficient using the Fermi golden rule. However, as temperature is raised, an interesting minimum is observed in the interband scattering, interpreted here as a result of the interplay between the tilt and the chemical potential increasing with temperature.

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arXiv2019

Band Gaps and Wavefunctions of Electrons Coupled to Pseudo Electromagnetic Waves in Rippled Graphene

The effects of a propagating sinusoidal out-of-plane flexural deformation in the electronic properties of a tense membrane of graphene are considered within a non-perturbative approach, leading to an electron-ripple coupling. The deformation is taken into account by introducing its corresponding pseudo-vector and pseudo-scalar potentials in the Dirac equation. By using a transformation to the time-cone of the strain wave, the Dirac equation is reduced to an ordinary second-order differential Matthieu equation, i.e., to a parametric pendulum, giving a spectrum of bands and gaps determined by resonance conditions between the electron and ripple wave-vector (G), and their incidence angles. The location of the nth gap is thus determined by $E\approx n v_{F}\hbar G$, where $v_F$ is the Fermi velocity. Physically, gaps are produced by diffraction of electrons in phase with the wave. The propagation is mainly in the direction of the ripple. In the case of a pure pseudoelectric field and for energies lower than a certain threshold, we found a different kind of equation. Its analytical solutions are in excellent agreement with the numerical solutions. The wavefunctions can be expressed in terms of the Matthieu cosine and sine functions, and for the case of a pure pseudo-electric potential, as a combination of Bessel functions.

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arXiv2019

Dynamical band gap tuning in Weyl semi-metals by intense elliptically polarized normal illumination and its application to $8-Pmmn$ borophene

The Dynamical-gap formation in Weyl semimetals modulated by intense elliptically polarized light is addressed through the solution of the time-dependent Schrödinger equation for the Weyl Hamiltonian via the Floquet theorem. The time-dependent wave functions and the quasi-energy spectrum of the two-dimensional Weyl Hamiltonian under normal incidence of elliptically polarized electromagnetic waves are obtained using a non-perturbative approach. In it, the Weyl equation is reduced to an ordinary second-order differential Mathieu equation. It is shown that the stability conditions of the Mathieu functions are directly inherited by the wave function resulting in a quasiparticle spectrum consisting of bands and gaps determined by dynamical diffraction and resonance conditions between the electron and the electromagnetic wave. Estimations of the electromagnetic field intensity and frequency, as well as the magnitude of the generated gap are obtained for the $8-Pmmn$ phase of borophene. We provide with a simple method that enables to predict the formation of dynamical-gaps of unstable wave functions and their magnitudes. This method can readily be adapted to other Weyl semimetals.

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arXiv2019

Phase Diagram for a model of Spin-Crossover in Molecular Crystals

Spin-crossover has a wide range of applications from memory devices to sensors. This has to do mainly with the nature of the transition, which may be abrupt, gradual or incomplete and may also present hysteresis. This transition alters the properties of a given sample, such as magnetic moment, color and electric resistance to name some. Yet, a thorough understanding of the phenomenon is still lacking. In this work a simple model is provided to mimic some of the properties known to occur in spin-crossover. A detailed study of the model parameters is presented using a mean field approach and exhaustive Monte Carlo simulations. A good agreement is found between the analytical results and the simulations for certain regions in the parameter-space. This mean field approach breaks down in parameter regions where the correlations and cooperativity may no longer be averaged over.

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arXiv2019

Viscoelasticity and dynamical gaps: rigidity in crystallization and glass-forming liquids

Rigidity plays an important role on the relaxation properties of glass forming melts, yet it is usually determined from the average co-ordination number through the chemical composition. A discussion is presented on how viscoelasticity can be used as an alternative way to determine glass rigidity and to give clues about the relaxation processes. In particular, it is shown that the transverse current dynamical structure factor of dense glass and crystal forming fluids contain rich information about rigidity that can be related with the presence of a dynamical-gap for transversal vibrational modes. As a result, the number of floppy modes can be related with the size of the dynamical gap and with the liquid relaxation time. From this, a dynamical average effective coordination number can be defined. As an example, numerical simulations for hard-disks in a dense fluid phase are provided. Finally, a discussion is presented on the need to improve glass viscoelasticity models to describe consistently non-exponential stress and strain relaxation.

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arXiv2019

Multi-flavor Dirac fermions in Kekulé-distorted graphene bilayers

Graphene's electronic structure can be fundamentally altered when a substrate- or adatom-induced Kekulé superlattice couples the valley and isospin degrees of freedom. Here, we show that the band structure of Kekulé-textured graphene can be re-engineered through layer stacking. We predict a family of Kekulé graphene bilayers that exhibit band structures with up to six valleys, and room-temperature Dirac quasiparticles whose masses can be tuned electrostatically. Fermi velocities half as large as in pristine graphene put this system in the strongly coupled regime, where correlated ground states can be expected.

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arXiv2019

How the overlap of excluded volume determines the configurational energy landscape and "thermodynamics" in the "one to five hard disks in a box" system

In this work, the effects of excluded volume are studied in the "one to five hard disks in a box" system. For one and two disks in different types of cages, the attractive and repulsive forces are calculated analytically. Attractive forces are due to excluded volume overlap, and thus allows to understand that in hard-core systems, second neighbors have an effective interaction, a fact that is usually neglected when considering only collisions with first-neighbors. The same effects are observed for numerical computations with five-disks in a box, as the distributions of distances between disks and disks to walls suggest. The results indicate that first and second-neighbor excluded volume interactions are essential to determine the energy landscape topology. This work supports previous observations that suggests that second-neighbor excluded volume interactions in hard-core systems are related with phase transitions.

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arXiv2018

Valley engineering by strain in Kekulé-distorted graphene

A Kekulé bond texture in graphene modifies the electronic band structure by folding the Brillouin zone and bringing the two inequivalent Dirac points to the center. This can result, in the opening of a gap (Kek-O) or the locking of the valley degree of freedom with the direction of motion (Kek-Y). We analyze the effects of uniaxial strain on the band structure of Kekulé-distorted graphene for both textures. Using a tight-binding approach, we introduce strain by considering the hopping renormalization and corresponding geometrical modifications of the Brillouin zone. We numerically evaluate the dispersion relation and present analytical expressions for the low-energy limit. Our results indicate the emergence of a Zeeman-like term due to the coupling of the pseudospin with the pseudomagnetic strain potential which separates the valleys by moving them in opposite directions away from the center of the Brillouin zone. For the Kek-O phase, this results in a competition between the Kekulé parameter that opens a gap and the magnitude of strain which closes it. While for the Kek-Y phase, in a superposition of two shifted Dirac cones. As the Dirac cones are much closer in the supperlattice reciprocal space that in pristine graphene, we propose strain as a control parameter for intervalley scattering.

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arXiv2018

Metal-Insulator transition in 8-Pmmn Borophene under perpendicular incidence of electromagnetic radiation

The energy spectrum for the problem of 8-Pmmn borophene's electronic carriers under perpendicular incidence of electromagnetic waves is studied without the use of any perturbative technique. This allows to study the effects of very strong fields. To obtain the spectrum and wavefunctions, the time-dependent Dirac equation is solved by using a frame moving with the space-time cone of the wave, i.e., by transforming the equation into an ordinary differential equation in terms of the wave-phase, leading to an electron-wave quasiparticle. The limiting case of strong fields is thus analyzed.The resulting eigenfunctions obey a generalized Mathieu equation,i.e., of a classical parametric pendulum. The energy spectrum presents bands, and a gap at the Fermi energy. The gaps are due to the space-time diffraction of electrons in phase with the electromagnetic field, i.e., electrons in borophene acquire an effective mass under strong electromagnetic radiation

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arXiv2018

Landauer-Büttiker conductivity for spatially-dependent uniaxial strained armchair-terminated graphene nanoribbons

The Landauer-Büttiker conductivity of arbitrary uniaxial spatially dependent strain in an armchair graphene nanoribbon is studied. Due to the uniaxial character of the strain, the corresponding transfer matrix can be reduced to a product of $2\times2$ matrices. Then the conductivity and the Fano factor can be calculated from this product. As an example of the technique, sinusoidal space dependent strain fields are studied using two different strain wavelengths. For the bigger wavelength the conductivity is reduced when compared with the unstrained case, although both conductivities are almost the same in shape. Whereas, for the smaller wavelength case, the conductivity is strongly modified. In spite of this, for energies close to the Dirac point energy, the conductivity and the Fano factor are quite similar to their unstrained counterpart for the two strain wavelengths here studied.

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arXiv2017

Topological phase-diagram of time-periodically rippled zigzag graphene nanoribbons

The topological properties of electronic edge states in time-periodically driven spatially-periodic corrugated zigzag graphene nanoribbons are studied. An effective one-dimensional Hamiltonian is used to describe the electronic properties of graphene and the time-dependence is studied within the Floquet formalism. Then the quasienergy spectrum of the evolution operator is obtained using analytical and numeric calculations, both in excellent agreement. Depending on the external parameters of the time-driving, two different kinds (type I and type II) of touching band points are found, which have a Dirac-like nature at both zero and $\pmπ$ quasienergy. These touching band points are able to host topologically protected edge states for a finite size system. The topological nature of such edge states was confirmed by an explicit evaluation of the Berry phase in the neighborhood of type I touching band points and by obtaining the winding number of the effective Hamiltonian for type II touching band points. Additionally, the topological phase diagram in terms of the driving parameters of the system was built.

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arXiv2017

Electronic and optical properties of strained graphene and other strained 2D materials: a review

This review presents the state of the art in strain and ripple-induced effects on the electronic and optical properties of graphene. It starts by providing the crystallographic description of mechanical deformations, as well as the diffraction pattern for different kinds of representative deformation fields. Then, the focus turns to the unique elastic properties of graphene, and to how strain is produced. Thereafter, various theoretical approaches used to study the electronic properties of strained graphene are examined, discussing the advantages of each. These approaches provide a platform to describe exotic properties, such as a fractal spectrum related with quasicrystals, a mixed Dirac-Schrödinger behavior, emergent gravity, topological insulator states, in molecular graphene and other 2D discrete lattices. The physical consequences of strain on the optical properties are reviewed next, with a focus on the Raman spectrum. At the same time, recent advances to tune the optical conductivity of graphene by strain engineering are given, which open new paths in device applications. Finally, a brief review of strain effects in multilayered graphene and other promising 2D materials like silicene and materials based on other group-IV elements, phosphorene, dichalcogenide- and monochalcogenide-monolayers is presented, with a brief discussion of interplays among strain, thermal effects, and illumination in the latter material family.

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arXiv2017

Topological edge states on time-periodically strained armchair graphene nanoribbons

We report the emergence of electronic edge states in time-periodically driven strained armchair terminated graphene nanoribbons. This is done by considering a short-pulse spatial-periodic strain field. Then, the tight-binding Hamiltonian of the system is mapped into a one dimensional ladder. The time periodicity is considered within the Floquet formalism. Thus the quasienergy spectrum is found numerically by diagonalizing the evolution operator. For some particular cases, the quasienergy spectrum is found analytically. We find that the system is able to support gapless and gapped phases. Very different edge states emerge for both the gapless and the gapful phases. In the case of the gapped phase, edge states emerge at the gap centered at zero quasienergy, although the Chern number is zero due to the chiral symmetry of the system. For the gapless phase, besides edge states at zero quasienergy, cosine like edge states which merge and coexist with the bulk band are observed. To confirm the topological nature of these edge states, we analytically obtained the effective Hamiltonian and its spectrum for a particular case, finding that the edge states are topologically weak. Finally, we found analytically the evolution of band edges and its crossings as a function of the driven period. Topological modes arise at such crossings.

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arXiv2017

Short time dynamics determine glass forming ability in a glass transition two-level model: a stochastic approach using Kramers' escape formula

The relationship between short and long time relaxation dynamics is obtained for a simple solvable two-level energy landscape model of a glass. This is done through means of the Kramers transition theory, which arises in very natural manner to calculate transition rates between wells. Then the corresponding stochastic master equation is analytically solved to find the population of metastable states. A relation between the cooling rate, the characteristic relaxation time and the population of metastable states is found from the solution of such equation. From this, a relationship between the relaxation times and the frequency of oscillation at the metastable states, i.e., the short time dynamics is obtained. Since the model is able to capture either a glass transition or a crystallization depending on the cooling rate, this gives a conceptual framework in which to discuss some aspects of rigidity theory.

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arXiv2017

Topological flat bands in time-periodically driven uniaxial strained graphene nanoribbons

We study the emergence of electronic non-trivial topological flat bands in time-periodically driven strained graphene within a tight binding approach based on the Floquet formalism. In particular, we focus on uniaxial spatially periodic strain since it can be mapped onto an effective one-dimensional system. Also, two kinds of time-periodic driving are considered: a short pulse (delta kicking) and a sinusoidal variation (harmonic driving). We prove that for special strain wavelengths, the system is described by a two level Dirac Hamiltonian. Even though the study case is gapless, we find that topologically non-trivial flat bands emerge not only at zero-quasienergy but also at $\pmπ$ quasienergy, the latter being a direct consequence of the periodicity of the Floquet space. Both kind of flat bands are thus understood as dispersionless bands joining two inequivalent touching band points with opposite Berry phase. This is confirmed by explicit evaluation of the Berry phase in the touching band points' neighborhood. Using that information, the topological phase diagram of the system is built. Additionally, the experimental feasibility of the model is discussed and two methods for the experimental realization of our model are proposed.

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arXiv2017

Tunable dichroism and optical absorption of graphene by strain engineering

We theoretically study the transmittance for normal incidence of linearly polarized light between two media separated by a strained graphene monolayer. We analytically characterize the degree of dichroism and the transparency of graphene as a function of an arbitrary uniform strain and the incident polarization. We discuss how measurements of dichroism and transparency for two different polarization directions can be used to determine the magnitude and direction of strain. Ours findings result in very useful tools to tune the graphene absorption by mechanical strain, as well as to design nano-devices to determine crack propagation in materials.

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arXiv2015

Effective Dirac Hamiltonian for anisotropic honeycomb lattices: optical properties

We derive the low-energy Hamiltonian for a honeycomb lattice with anisotropy in the hopping parameters. Taking the reported Dirac Hamiltonian for the anisotropic honeycomb lattice, we obtain its optical conductivity tensor and its transmittance for normal incidence of linearly polarized light. Also, we characterize its dichroic character due to the anisotropic optical absorption. As an application of our general findings, which reproduce the case of uniformly strained graphene, we study the optical properties of graphene under a nonmechanical distortion.

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arXiv2015

Minimal cooling speed for glass transition in a simple solvable energy landscape model

The minimal cooling speed required to form a glass is obtained for a simple solvable energy landscape model. The model, made from a two-level system modified to include the topology of the energy landscape, is able to capture either a glass transition or a crystallization depending on cooling rate. In this setup, the minimal cooling speed to achieve glass formation is then found to be related with the relaxation time and with the thermal history. In particular, we obtain that the thermal history encodes small fluctuations around the equilibrium population which are exponentially amplified near the glass transition, which mathematically corresponds to the boundary layer of the master equation. Finally, to verify our analytical results, a kinetic Monte-Carlo simulation was implemented.

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