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H. Vivas

Publications and source records attributed to H. Vivas.

4 recordsLinked to original sources

A PDE approach to the existence and regularity of surfaces of minimum mean curvature variation

We develop an analytic theory of existence and regularity of surfaces (given by graphs) arising from the geometric minimization problem $$\min_{\mathcal{M}}\frac{1}{2}\int_{\mathcal{M}}|\nabla_{\mathcal{M}}H|^2\,dA$$ where $\mathcal{M}$ ranges over all $n$-dimensional manifolds in $\mathbb{R}^{n+1}$ with prescribed boundary, $\nabla_{\mathcal{M}}H$ is the tangential gradient along $\mathcal{M}$ of the mean curvature $H$ of $\mathcal{M}$ and $dA$ is the differential of surface area. The minimizers, called surfaces of minimum mean curvature variation, are central in applications of computer-aided design, computer-aided manufacturing and mechanics. Our main results show the existence of both smooth surfaces and of variational solutions to the minimization problem together with geometric regularity results. These are the first analytic results available on the literature for this problem.

math.DG

Magnetic Stability for the Hatree-Fock Ground State in Two Dimensional Rashba-Gauge Electronic Systems

The magnetic Hartree Fock ground state stability for a two-dimensional interacting electron system with Rashba-type coupling is studied by implementing the standard many body Green's function formalism. The externally applied electrical field $\mathbf{E}$ enters into the Hamiltonian model through a local gauge-type transformation $\sim E_{j}\mathcal{A}_{j}(\mathbf{k})$, with $\mathcal{A}_{j}(\mathbf{k})$ as the spin gauge vector potential. Phase diagrams associated to the average spin polarization, the Fermi energy, electron density and energy band gap at zero temperature are constructed. The magnetic polarization state as a function of $\mathbf{E}$ is obtained by minimizing the Helmholtz energy functional $\mathcal{F}$ with respect to the average $Z$-spin: $\langle\hatσ_{Z}\rangle$. We have found that the electric field might reverse the magnetic ground state, unlike the characteristic decreasing associated to the linear $\hat{\boldsymbolσ}\cdot(\mathbf{k\times E})$ spin-momentum-field coupling.

cond-mat.str-el

Anomalous Magnetism for Dirac Electrons in Two Dimensional Rashba Systems

Spin-spin correlation function response in the low electronic density regime and externally applied electric field is evaluated for 2D metallic crystals under Rashba-type coupling, fixed number of particles and two-fold energy band structure. Intrinsic Zeeman-like effect on electron spin polarization, density of states, Fermi surface topology and transverse magnetic susceptibility are analyzed in the zero temperature limit. A possible magnetic state for Dirac electrons depending on the zero field band gap magnitude under this conditions is found.

cond-mat.str-el

Path Integral Treatment of Fluctuations in the BCS-BEC Crossover in Superfluid Fermi Gases

By using a Path Integral formalism, we study the fluctuating fields in the order parameter for a 3D uniform Fermi gas at T<Tc. The BCS-BEC crossover condition is included into the analysis in the framework of the usual s-wave scattering approximation. The general expressions for the temperature dependence of the partition function and the quadratic average in the fluctuations fields are explicitly established in terms of the BCS-Gor'kov correlation propagators.

cond-mat.str-el