Brazilian Journal of Physics最新文献

Electron spin resonance (ESR) dating is currently one of the major dating techniques in archaeology, paleoanthropology, and paleontology. Years of experience have led to some standard protocols for ESR dating. However, these limit studies because some tooth enamel samples fail the criteria in the protocols. One criterion is that the enamel must be completely free of dentine. There are tooth fragments that are so small that complete cleaning would not leave enough enamel for dating. To see if this standard protocol is absolutely required, several samples of “dirty” enamel were compared. The results confirm the need for essentially complete cleaning.

In the last decades, to improve the CAP treatment efficiency, its biological effects in combination with other physical modalities have widely investigated. However, the physical insight into most of supposed synergistic effects remained elusive. In this regard, the synergetic effect of cold plasma and magnetic field has been used for different applications, especially due to considerable synergistic in biological media reactivity. In the present paper, using a 420 mT N42 magnet, the effect of the perpendicular external static magnetic field (SMF) on the cold atmospheric pressure plasma (CAP) characteristics, such as electron temperature and density, is investigated based on the optical emission spectroscopy, utilizing the Boltzmann plot method, Saha-Boltzmann equation, and Specair software simulation. Results showed that the rotational and electronic excitational temperatures experienced 100 K and 550 K increases in the presence of SMF, respectively. In contrast, the vibrational and translational temperatures remained constant. Moreover, electron temperature was estimated as 1.04 eV in the absence of SMF and increased up to 1.24 eV in the presence of SMF. In addition, the Saha-Boltzmann equation illustrated that the electron density increased in the presence of the additional SMF. The results of the present study indicated that the magnetic field could be an assistant to the cold plasma effect, beneficial in medical applications due to modifications in plasma temperature and electron density.

Linear dichroism is a novel method for texture analysis and is used to measure the order parameter of liquid crystals. It determines the dichroic ratio by analyzing the maximum absorption of textural intensities polarized parallel and perpendicular to the optical axis. To capture liquid crystal textures as a function of temperature, a polarized optical microscope with an attached camera is employed under parallel and crossed polarizer conditions. This analysis adheres to the Beer-Lambert law, relating transmitted and absorbed textural intensities in liquid crystals. Here, we investigate nematic liquid crystals, specifically 4-cyano-4′-pentylbiphenyl and p-n-alkyl benzoic acid (where n = 8), using texture analysis to study their physical characteristics.

The vibrational spectrum determines, to a large extent, a material’s thermal expansion. However, extracting information on the material’s vibrational spectrum from the measured thermal expansion is an ill-conditioned inverse problem whose solution generally involves regularization techniques. In this work, the procedure for extracting the vibrational density of states-weighted Grüneisen parameter spectra ((G_{a}(Theta )) and (G_{c}(Theta ))) from the temperature-dependent linear thermal expansion coefficients parallel to the crystallographic a- and c-axis of materials with a hexagonal Bravais lattice is described and then applied to graphite. The resulting spectra, obtained by solving an inverse problem consisting of a pair of coupled Fredholm integrals of the first kind, exhibit two main contributions to (G_{a}(Theta )), centered around (Theta) = 100 K and (Theta) = 1200 K, and one narrow peak in (G_{c}(Theta )) centered around (Theta) = 100 K. The low-energy, narrow peak of negative amplitude in (G_{a}(Theta )) is in good agreement with a large and negative Grüneisen parameter previously reported for transverse acoustic modes in graphite and accounts for the negative thermal expansion exhibited by graphite along the a-axis up to about 655 K. A simplified vibrational density of states-weighted Grüneisen parameter spectra of graphite, consisting of the two lowest energy, narrow peaks in (G_{a}(Theta )) and (G_{c}(Theta )), plus an asymmetric Gaussian peak with positive amplitude in (G_{a}(Theta )), is proposed which still captures the main features of graphite’s thermal expansion.

The Kuramoto model, describing the synchronization dynamics of coupled oscillators, has been generalized in many ways over the past years. One recent extension of the model replaces the oscillators, originally characterized by a single phase, by particles with (D-1) internal phases, represented by a point on the surface of the unit D-sphere. Particles are then more easily represented by D-dimensional unit vectors than by (D-1) spherical angles. However, numerical integration of the state equations should ensure that the propagated vectors remain unit and that particles rotate on the sphere as predicted by the dynamical equations. As discussed in (Lee et al. in Journal of Statistical Mechanics: Theory and Experiment 2023(4):043403, 2023), integration of the three-dimensional Kuramoto model using Euler’s method with time step (Delta t) not only changes the norm of the vectors but produces a small rotation of the particles around the wrong axis. Importantly, the error in the axis’ direction does not vanish in the limit (Delta t rightarrow 0). Therefore, instead of displacing the unit vectors in the direction of the velocity, one should perform a sequence of direct small rotations, as dictated by the equations of motion. This keeps the particles on the sphere at all times, ensuring exact norm preservation, and rotates the particles around the proper axis for small (Delta t) (Lee et al. in Journal of Statistical Mechanics: Theory and Experiment 2023(4):043403, 2023). Here, I propose an alternative way to do such integration by rotations in 3D that can be generalized to more dimensions using Cayley-Hamilton’s theorem. Explicit formulas are provided for 2, 3, and 4 dimensions. I also compare the results with the fourth-order Runge–Kutta method, which seems to provide accurate results even requiring renormalization of the vectors after each integration step.