Search

We investigate theoretically and experimentally the optical second harmonic generation (SHG) with a twisted Gaussian Schell model (TGSM) beam as the fundamental field. We use Type-II phase matching and analyze the cross spectral density (CSD) of the SHG output beam when the input fundamental is prepared with a TGSM structure. We analyze two synthetization methods for preparing the TGSM fundamental beam and we find that for one method the SHG is also a TGSM beam. For the other method, we find that the SHG is not a TGSM beam and presents an anomalous CSD possessing a dip instead of a peak in the transverse spatial structure. Moreover, we show that the dip depth is directly related to the twisted phase parameter, being absent for a non twisted GSM beam. Our results show that the SHG from a fundamental TGSM beam can result in a doubled frequency TGSM or in a non-TGSM beam depending on the synthetization method.

We experimentally demonstrate resonance of first-order vector vortex beams (VVB) with a triangular optical cavity. We also show that, due to their symmetry properties, the so-called radial and azimuthal VVBs do not resonate at the same cavity length, which could be explored to use the triangular resonator as a mode sorter. In addition, an intracavity Pancharatnam phase shifter (PPS) is implemented in order to compensate for any birefringent phase that the cavity mirrors may introduce.

The effects of the Ohmic and magnetic density currents are investigated in the linearized Euler-Heisenberg electrodynamics. The linearization is introduced through an external magnetic field, in which the vector potential of the Euler-Heisenberg electrodynamics is expanded around of a magnetic background field, that we consider uniform and constant in this paper. From the Euler-Heisenberg linearized equations, we obtain the solutions for the refractive index associated with the electromagnetic wave superposition, when the current density is ruled by the Ohm law, and in the second case, when the current density is set by a isotropic magnetic conductivity. These solutions are functions of the magnetic background $({\bf B})$, of the wave propagation direction $({\bf k})$, it also depends on the conductivity, and on the wave frequency. As consequence, the dispersion and the absorption of plane waves change when ${\bf B}$ is parallel to ${\bf k}$ in relation to the case of ${\bf B}$ perpendicular to ${\bf k}$ in the medium. The characteristics of the refraction index related to directions of ${\bf B}$ and of the wave polarization open a discussion for the birefringence in this medium., Comment: 9 pages, 6 figures, published version in IJMPA

A study of double charmed meson production in proton-proton and proton-nucleus collisions at the LHC energies is performed. Based on the color dipole formalism developed in the transverse momentum representation and the double parton scattering mechanism, predictions are made for the transverse momentum differential cross section for different pairs of $D$-mesons. The theoretical results consider the center-of-mass energy and forward rapidities associated to the measurements by the LHCb Collaboration. The results considering different unintegrated gluon distributions are presented and compared to data and predictions for proton-nucleus collisions are provided., Comment: 20 pages, 4 figures

Accelerated light has been demonstrated with laser light and diffraction. Within the diffracting field it is possible to identify a portion that carries most of the beam energy, which propagates in a curved trajectory as it would have been accelerated by a gravitational field for instance. Here, we analyze the effects of this kind of acceleration over the entanglement between twin beams produced in spontaneous parametric down-conversion. Our results show that acceleration does not affect entanglement significantly, under ideal conditions. The optical scheme introduced can be useful in the understanding of processes in the boundary between gravitation and quantum physics., Comment: 6 pages, 3 figures

We discuss the full optical response of twisted bilayer graphene at the neutrality point close to the magic angle within the continuum model (CM). Firstly, we identify three different channels consistent with the underlying $D_3$ symmetry, yielding the total, magnetic, and chiral response. Secondly, we numerically calculate the full optical response in the immediate vicinity of the magic angle $\theta_m$ which provides a direct mapping of the CM onto an effective two-band model. We, further, show that the ground-state of the CM in the immediate vicinity of $\theta_m$ is unstable towards transverse current fluctuations, a so-called Condon instability. Thirdly, due to the large counterflow, the acoustic plasmonic excitations with typical wave numbers have larger energies than the optical ones and their energy density may be largely enhanced at certain frequencies which we denominate as {\it chiral resonances}. Finally, we discuss symmetry relations for the optical response and their consequences for the chiral response., Comment: 19 pages, 7 figures

A triangular-lattice pattern is observed in light beams resulting from the spatial cross modulation between an optical vortex and a triangular shaped beam undergoing parametric interaction. Both up- and down-conversion processes are investigated, and the far-field image of the converted beam exhibits a triangular lattice. The number of sites and the lattice orientation are determined by the topological charge of the vortex beam. In the down-conversion process, the lattice orientation can also be affected by phase conjugation. The observed cross modulation works for a large variety of spatial field structures, and could replace solid-state devices at wavelengths where they are not yet available.

We use spin-lattice dynamics simulations to study the possibility of modeling the magnetic hysteresis behavior of a ferromagnetic material. The temporal evolution of the magnetic and mechanical degrees of freedom is obtained through a set of two coupled Langevin equations. Hysteresis loops are calculated for different angles between the external field and the magnetocrystalline anisotropy axes. The influence of several relevant parameters is studied, including the field frequency, magnetic damping, magnetic anisotropy (magnitude and type), magnetic exchange, and system size. The role played by a moving lattice is also discussed. For a perfect bulk ferromagnetic system we find that, at low temperatures, the exchange and lattice dynamics barely affect the loops, while the field frequency and magnetic damping have a large effect on it. The influence of the anisotropy magnitude and symmetry are found to follow the expected behavior. We show that a careful choice of simulation parameters allows for an excellent agreement between the spin-lattice dynamics measurements and the paradigmatic Stoner-Wohlfarth model. Furthermore, we extend this analysis to intermediate and high temperatures for the perfect bulk system and for spherical nanoparticles, with and without defects, reaching values close to the Curie temperature. In this temperature range, we find that lattice dynamics has a greater role on the magnetic behavior, especially in the evolution of the defective samples. The present study opens the possibility for more accurate inclusion of lattice defects and thermal effects in hysteresis simulations

The $D$-meson production is investigated by considering the unintegrated gluon distribution within the dipole approach in the momentum representation. We analyze the $D$-meson spectrum accounting for the effects of nonlinear behavior of the QCD dynamics which can be accordingly addressed in the dipole framework. The unintegrated gluon distribution is obtained by using geometric scaling property and the results are compared to the Glauber-Gribov framework. The absolute transverse momentum spectra and the nuclear modification ratios are investigated. Predictions are compared with the experimental measurements by the ALICE and LHCb Collaborations in $pA$ collisions for different rapidity bins., Comment: 23 pages, 9 figures, 1 table

The possible time variation of the fundamental constants of nature has been an active subject of research since the large-number hypothesis was proposed by Dirac. In this paper, we propose a new method to investigate a possible time variation of the speed of light ($c$) along with the fine-structure constant ($\alpha$) using Strong Gravitational Lensing (SGL) and Type Ia Supernovae (SNe Ia) observations. We assume a general approach to describe the mass distribution of lens-type galaxies, the one in favor of the power-law index model (PLAW). We also consider the runaway dilaton model to describe a possible time-variation of $\alpha$. In order to explore the results deeply, we split the SGL sample into five sub-samples according to the lens stellar velocity dispersion and three sub-samples according to lens redshift. The results suggest that it is reasonable to treat the systems separately, but no strong indication of varying $c$ was found., Comment: 10 Pages, 4 Figures, and 1 Table. Published in JCAP

The production of $D$-mesons in high-energy $pp$ collisions at the LHC kinematic regime is analyzed with the dipole approach in the momentum representation. We present predictions for the $D$-meson differential cross section in terms of the transverse momentum and rapidity distributions taking into account the nonlinear behavior of the QCD dynamics. Comparison between our results and the corresponding experimental measurements reported by the ALICE and LHCb Collaborations in different rapidity bins is performed. We show that the $D$-meson production in the high energy limit can be properly addressed by using the QCD dipole transverse momentum distributions., Comment: 16 pages, 5 figures

We discuss novel transverse plasmon-polaritons that are hosted by AA- and AB-stacked bilayer graphene due to perfect nesting. They are composed of oscillating counterflow currents in between the layers, giving a clear interpretation for these collective modes as magnetic excitations carrying magnetic moment parallel to the planes. For AA-stacked bilayer graphene, these modes can reach zero frequency at the neutrality point and we thus predict a symmetry broken ground-state leading to in-plane orbital ferromagnetism. Even though it could be hard to detect them in real solid-state devices, these novel magnetic plasmons should be observable in artificial set-ups such as optical lattices. Also, our results might be relevant for magic angle twisted bilayer graphene samples as their electronic properties are mostly determined by confined AA-stacked regions., Comment: 12 pages, 6 figures

We demonstrate that a triangular cavity can operate as an efficient discriminator for Hermite-Gaussian optical modes. The process is applied to distinct fields produced by a Spatial Light Modulator, decomposing the generated paraxial field onto a basis of modes defined by the resonant modes of the optical cavity. We also present a proposal for the realization of a mode sorter based on a sequence of these cavities, without the need for optical isolation. We discuus its possible applications in sensitive measurements and distribution of information in optical systems. The high purity of the process should enable the use at the level of quantum information sharing., Comment: 9 pages, 9 figures

An analysis of prompt photon production in high energy nuclear collisions at the LHC energy regime is performed within the parton saturation picture taking into account the updated phenomenological color dipole models. The results are confronted with the measurements made by the ALICE, ATLAS, and CMS experiments in terms of photon transverse momentum at different rapidity bins. As a result, we show that the prompt photon production exhibits distinct scalings in $AA$ events associated to geometrical properties of the collision and can be properly addressed in the color dipole formalism., Comment: 17 pages, 5 figures

We analyze how a quantum-gravity-induced change in the number of thermal dimensions (through a modified dispersion relation) affects the geometry and the thermodynamics of a charged black hole. To that end we resort to Kiselev's solution as the impact such modifications have on the evaporation rate of the black hole becomes more clear. As an application, we study the case for which the thermal dimension is reduced to two., Comment: 6 pages, 2 figures. Updated to match published version

It is shown that chiral plasmons, characterized by a longitudinal magnetic moment accompanying the longitudinal charge plasmon, lead to electromagnetic near-fields that are also chiral. For twisted bilayer graphene, we estimate that the near field chirality of screened plasmons can be several orders of magnitude larger than that of the related circularly polarized light. The chirality also manifests itself in a deflection angle that is formed between the direction of the plasmon propagation and its Poynting vector. Twisted van der Waals heterostructures might thus provide a novel platform to promote enantiomer-selective physio-chemical processes in chiral molecules without the application of a magnetic field or external nano-patterning that break time-reversal, mirror plane or inversion symmetry, respectively., Comment: 33 pages, 5 figures

The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics (SD) simulations coupled to molecular dynamics (MD-SD), using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom M(T), including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on M(T) are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering (SRMO). Analytical approximations to the magnetization as a function of temperature and size are proposed.

Prompt photon production in hadronic collisions at the RHIC and the LHC energies is investigated within the QCD color dipole approach. Predictions for the nuclear modification factor in $pA$ collisions are evaluated based on parton saturation framework and the results are compared to the experimental measurements as a function of the photon transverse momentum at different rapidity bins. The reliability of the models is performed with the data from PHENIX, ATLAS, and ALICE Collaborations. Moreover, we show that the observed $x_T$-scaling of prompt photon production in $pp$ and $pA$ collisions can positively be addressed in the QCD color dipole formalism., Comment: 17 pages, 10 figures

We derive a simple formula for the real-space chirality of twisted bilayer graphene that can be related to the cross-product of its sheet currents. This quantity shows well-defined plateaus for the first remote band as function of the gate voltage which are approximately quantized for commensurate twist angles. The zeroth plateau corresponds to the first magic angle where a sign change occurs due to an emergent $C_6$-symmetry. Our observation offers a new definition of the magic angle based on a macroscopic observable which is accessible in typical transport experiments., Comment: 5 pages, 4 figures

A study on the prompt photon production within the QCD color dipole picture with emphasis in $pp$ and $pA$ collisions at the LHC energy regimes is performed. We present predictions for the differential cross section as a function of photon transverse momentum at different rapidity bins considering updated phenomenological color dipole models, which take into account the QCD gluon saturation physics. The results are directly compared to the recent experimental measurements provided by CMS and ATLAS Collaborations, showing a reasonable agreement in all rapidity bins with no free parameters. Special attention is given to the IPSAT model given its good description of the data in all rapidity bins from low- to high-$p_{T}$ ranges. As a result, a free-parameter approach has succeeded in describing the LHC data for prompt photon production, while new predictions for the 13-TeV data is presented in view of new data to confirm such prospect., Comment: 12 pages, 18 figures

By using a non-local, quantum mechanical response function we study graphene plasmons in a one-dimensional superlattice (SL) potential $V_0 \cos G_0x$. The SL introduces a quantum energy scale $E_G \sim \hbar v_F G_0$ associated to electronic sub-band transitions. At energies lower than $E_G$, the plasmon dispersion is highly anisotropic; plasmons propagate perpendicularly to the SL axis, but become damped by electronic transitions along the SL direction. These results question the validity of semiclassical approximations for describing low energy plasmons in periodic structures. At higher energies, the dispersion becomes isotropic and Drude-like with effective Drude weights related to the average of the absolute value of the local chemical potential. Full quantum mechanical treatment of the kinetic energy thus introduces non-local effects that delocalize the plasmons in the SL, making the system behave as a meta-material even near singular points where the charge density vanishes., Comment: 22 pages, 14 figures

In this paper we present a comprehensive analysis of the exclusive heavy quark photoproduction in $pp$, $pPb$ and $PbPb$ collisions at LHC and FCC energies using the color dipole formalism and taking into account of nonlinear corrections to the QCD dynamics. We estimate the rapidity distributions and cross sections for the charm and bottom production considering three phenomenological models for the dipole-proton scattering amplitude that are able to describe the $ep$ HERA data. Our results indicate that a future experimental analysis of this process is feasible, which will allow us to improve our understanding of the QCD dynamics., Comment: 9 pages, 6 figures, 2 tables. Improved version published in Nuclear Physics A

The particle production at very forward rapidities is expected to be sensitive to the non-linear effects in the QCD dynamics at high energies. In this paper we present, for the first time, the predictions of the Color Dipole formalism for the single inclusive jet photoproduction in $pp$ and $pPb$ collisions considering the very forward rapidities probed by the CMS-CASTOR calorimeter, which will be characterized by a jet in the rapidity range of $5.2 \le Y \le 6.6$, a rapidity gap in the rapidity range probed by the central CMS detector and one of the incident hadrons remaining intact in the final state. The transverse momentum distributions are estimated considering the more recent phenomenological models for the dipole-proton scattering amplitude, which are based on the Color Glass Condensate formalism and are able to describe the inclusive and exclusive $ep$ HERA data. Our results indicate that a future experimental analysis of this process is, in principle, feasible and useful to constrain the description of the QCD dynamics at high energies., Comment: 9 pages, 5 figures. arXiv admin note: text overlap with arXiv:1711.04497

The transport properties of a twisted bilayer graphene barrier are investigated for various twist angles. Remarkably, for small twist angles around the magic angle $\theta_m \sim 1.05^{\circ}$, the local currents around the AA-stacked regions are strongly enhanced compared to the injected electron rate. Furthermore, the total and counterflow (magnetic) current patterns show high correlations in these regions, given rise to well-defined magnetic moments that form a magnetic Moir\'e superlattice. The orientation and magnitude of these magnetic moments changes as function of the gate voltage and possible implications for emergent spin-liquid behaviour are discussed., Comment: 18 pages, 20 figures

We present the exact solution of a family of two-spins models. The models are solved by the algebraic Bethe ansatz method using the $gl(2)$-invariant $R$-matrix and a multi-spins Lax operator. The interactions are by the Heisenberg spins exchange. We are also considering magnetic $B$-fields and a term for Haldane single spin anisotropy., Comment: 15 pages, 2 figures. arXiv admin note: text overlap with arXiv:1605.08452

Based upon the intrinsic symmetries approach to inhomogeneous cosmologies, we propose an exact solution to Einstein's field equations where the spatial sections are flat and the source is a non-perfect fluid such that the dissipative terms can be written in terms of spatial gradients of the energy density under a suitable choice of the coordinate system. It is shown through the calculation of the luminosity distance as a function of the redshift that the presence of such inhomogeneities may lead to an effective deceleration parameter compatible with either the standard $\Lambda$CDM model or LTB models depending on the choice of boundary conditions with no exotic matter. This fact is another evidence that different inhomogeneous models should be carefully investigated in order to verify which model may be compatible with observations and still be as close as possible to the standard model regarding the underlying assumptions, without resorting necessarily to exotic matter components., Comment: 12 pages, last version, published in IJMPD

We investigate the field and spin-momentum coupling of edge plasmons hosted by general two-dimensional materials and identify sweet spots depending on the polarisation plane, ellipticity and the position of an electric dipole relative to the plane and edge. Exciting the dipole at these sweet spots by propagating light leads to uni-directional propagating edge plasmons or edge modes are totally suppressed. We also extent previous approximate treatments [A. Fetter Phys. Rev. B 32, 7676 (1985)] to include anisotropy and hyperbolic systems, elucidating its predictions for the existence of edge modes. A thorough assessment of the approximate description is carried out, comparing its spin-momentum coupling features in the near field with exact results from Wiener-Hopf techniques. Simulations are also performed confirming the overall picture. Our results shed new light on the quest of chiral plasmonics in 2D materials and should be relevant for future experiments., Comment: 11 pages, 6 figures

We considered the Asynchronous SIR (susceptible-infected-removed) model on Penrose and Ammann-Beenker quasiperiodic lattices, and obtained its critical behavior by using Newman-Ziff algorithm to track cluster propagation by making a tree structure of clusters grown at the dynamics, allowing to simulate SIR model on non-periodic lattices and measure any observable related to percolation. We numerically calculated the order parameter, defined in a geographical fashion by distinguish between an epidemic state, characterized by a spanning cluster formed by the removed nodes and the endemic state, where there is no spanning cluster. We obtained the averaged mean cluster size which plays the role of a susceptibility, and a cumulant ratio defined for percolation to estimate the epidemic threshold. Our numerical results suggest that the system falls into two-dimensional dynamic percolation universality class and the quasiperiodic order is irrelevant, in according to results for classical percolation., Comment: 16 pages, 5 figures, 43 cited references