Search results for "Formalism"
showing 10 items of 357 documents
Expressions of Effective Hamiltonian Parameters of XY4 Molecules in the Tetrahedral Formalism
1998
We have derived expressions of second-order effective Hamiltonian parameters of XY4 molecules in the tetrahedral formalism (1992, J. P. Champion et al., "Spectroscopy of the Earth's Atmosphere and Interstellar Medium: Spherical Top Spectra," Academic Press, San Diego). They are written as a function of the force constants of the potential expanded in terms of the dimensionless normal coordinates. These expressions can be used in the isolated band scheme as well as in the polyad one. The ambiguity of the effective Hamiltonian parameters is treated. Relations between the parameters for q2 and q4 terms and Hecht's anharmonicity constants (1960, K. T. Hecht, J. Mol. Spectrosc. 5, 355-389) in th…
Gravitational lensing on the Cosmic Microwave Background by gravity waves
1997
We study the effect of a stochastic background of gravitational waves on the gravitational lensing of the Cosmic Microwave Background (CMB) radiation. It has been shown that matter density inhomogeneities produce a smoothing of the acoustic peaks in the angular power spectrum of the CMB anisotropies. A gravitational wave background gives rise to an additional smoothing of the spectrum. For the most simple case of a gravitational wave background arising during a period of inflation, the effect results to be three to four orders of magnitude smaller than its scalar counterpart, and is thus undetectable. It could play a more relevant role in models where a larger background of gravitational wa…
Improved description of the pion-nucleon scattering phenomenology in covariant baryon chiral perturbation theory
2014
We highlight some of the recent advances in the application of chiral effective field theory (chiral EFT) with baryons to the $\pi N$ scattering process. We recall some problems that cast doubt on the applicability of chiral EFT to $\pi N$ and show how the relativistic formalism, once the $\Delta(1232)$-resonance is included as an explicit degree of freedom, solves these issues. Finally it is shown how this approach can be used to extract the $\sigma$-terms from phenomenological information.
Effect of magnons on the temperature dependence and anisotropy of spin-orbit torque
2020
We investigate the influence of magnons on the temperature-dependence and the anisotropy of the spin-orbit torque (SOT). For this purpose we use 3rd order perturbation theory in the framework of the Keldysh formalism in order to derive suitable equations to compute the magnonic SOT. We find several contributions to the magnonic SOT, which depend differently on the spin-wave stiffness $\mathcal{A}$ and on the temperature $T$, with the dominating contribution scaling like $T^{2}/\mathcal{A}^{2}$. Based on this formalism we compute the magnonic SOT in the ferromagnetic Rashba model. For large Rashba parameters the magnonic SOT is strongly anisotropic and for small quasiparticle broadening it m…
First-principles and semi-empirical calculations for bound hole polarons in KNbO3
1999
The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Intermediate Neglect of the Differential Overlap (INDO) based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used, respectively. We predict existence of both, one-site and two-site (molecular) polarons with close optical absorption energies (0.9 eV and 0.95 eV). The relevant experimental data are discussed.
Spin accumulation from nonequilibrium first principles methods
2021
For the technologically relevant spin Hall effect, most theoretical approaches rely on the evaluation of the spin-conductivity tensor. In contrast, for most experimental configurations the generation of spin accumulation at interfaces and surfaces is the relevant quantity. Here, we directly calculate the accumulation of spins due to the spin Hall effect at the surface of a thin metallic layer, making quantitative predictions for different materials. Two distinct limits are considered, both relying on a fully relativistic Korringa-Kohn-Rostoker density functional theory method. In the semiclassical approach, we use the Boltzmann transport formalism and compare it directly with a fully quantu…
Becke-Johnson-type exchange potential for two-dimensional systems
2009
We extend the Becke-Johnson approximation [J. Chem. Phys. 124, 221101 (2006)] of the exchange potential to two dimensions. We prove and demonstrate that a direct extension of the underlying formalism may lead to divergent behavior of the potential. We derive a cure to the approach by enforcing the gauge invariance and correct asymptotic behavior of the exchange potential. The procedure leads to an approximation which is shown, in various quasi-two-dimensional test systems, to be very accurate in comparison with the exact exchange potential, and thus a considerable improvement over the commonly applied local-density approximation.
Dynamics of noncollinear antiferromagnetic textures driven by spin current injection
2019
We present a theoretical formalism to address the dynamics of textured, noncolliear antiferromagnets subject to spin current injection. We derive sine-Gordon type equations of motion for the antiferromagnets, which are applicable to technologically important antiferromagnets such as Mn3Ir and Mn3Sn, and enables an analytical approach to domain wall dynamics in those materials. We obtain the expression for domain wall velocity, which is estimated to reach around 1 km/s in Mn3Ir by exploiting spin Hall effect with electric current density around 10^11A/m^2.
On the sources of the late integrated Sachs-Wolfe effect
2000
In some scenarios, the peculiar gravitational potential of linear and mildly nonlinear structures depends on time and, as a result of this dependence, a late integrated Sachs-Wolfe effect appears. Here, an appropriate formalism is used which allows us to improve on the analysis of the spatial scales and locations of the main cosmological inhomogeneities producing this effect. The study is performed in the framework of the currently preferred flat model with cosmological constant, and it is also developed in an open model for comparisons. Results from this analysis are used to discuss the contribution of Great Attractor-like objects, voids, and other structures to the CMB anisotropy.
Thermodynamic formalism for transport coefficients with an application to the shear modulus and shear viscosity.
2016
We discuss Onsager's thermodynamic formalism for transport coefficients and apply it to the calculation of the shear modulus and shear viscosity of a monodisperse system of repulsive particles. We focus on the concept of extensive "distance" and intensive "field" conjugated via a Fenchel-Legendre transform involving a thermodynamic(-like) potential, which allows to switch ensembles. Employing Brownian dynamics, we calculate both the shear modulus and the shear viscosity from strain fluctuations and show that they agree with direct calculations from strained and non-equilibrium simulations, respectively. We find a dependence of the fluctuations on the coupling strength to the strain reservoi…