
Asenjo, F. A., & Comisso, L. (2015). Generalized Magnetofluid Connections in Relativistic Magnetohydrodynamics. Phys. Rev. Lett., 114(11), 5 pp.
Abstract: The concept of magnetic connections is extended to nonideal relativistic magnetohydrodynamical plasmas. Adopting a general set of equations for relativistic magnetohydrodynamics including thermalinertial, thermal electromotive, Hall, and currentinertia effects, we derive a new covariant connection equation showing the existence of generalized magnetofluid connections that are preserved during the dissipationless plasma dynamics. These connections are intimately linked to a general antisymmetric tensor that unifies the electromagnetic and fluid fields, allowing the extension of the magnetic connection notion to a much broader concept.



Asenjo, F. A., & Comisso, L. (2017). Relativistic Magnetic Reconnection in Kerr Spacetime. Phys. Rev. Lett., 118(5), 5 pp.
Abstract: The magnetic reconnection process is analyzed for relativistic magnetohydrodynamical plasmas around rotating black holes. A simple generalization of the SweetParker model is used as a first approximation to the problem. The reconnection rate, as well as other important properties of the reconnection layer, has been calculated taking into account the effect of spacetime curvature. Azimuthal and radial current sheet configurations in the equatorial plane of the black hole have been studied, and the case of small black hole rotation rate has been analyzed. For the azimuthal configuration, it is found that the black hole rotation decreases the reconnection rate. On the other hand, in the radial configuration, it is the gravitational force created by the black hole mass that decreases the reconnection rate. These results establish a fundamental interaction between gravity and magnetic reconnection in astrophysical contexts.



Baudin, K., Fusaro, A., Krupa, K., Garnier, J., Rica, S., Millot, G., et al. (2020). Classical RayleighJeans Condensation of Light Waves: Observation and Thermodynamic Characterization. Phys. Rev. Lett., 125(24), 244101.
Abstract: Theoretical studies on wave turbulence predict that a purely classical system of random waves can exhibit a process of condensation, which originates in the singularity of the RayleighJeans equilibrium distribution. We report the experimental observation of the transition to condensation of classical optical waves propagating in a multimode fiber, i.e., in a conservative Hamiltonian system without thermal heat bath. In contrast to conventional selforganization processes featured by the nonequilibrium formation of nonlinear coherent structures (solitons, vortices, ...), here the selforganization originates in the equilibrium RayleighJeans statistics of classical waves. The experimental results show that the chemical potential reaches the lowest energy level at the transition to condensation, which leads to the macroscopic population of the fundamental mode of the optical fiber. The nearfield and farfield measurements of the condensate fraction across the transition to condensation are in quantitative agreement with the RayleighJeans theory. The thermodynamics of classical wave condensation reveals that the heat capacity takes a constant value in the condensed state and tends to vanish above the transition in the normal state. Our experiments provide the first demonstration of a coherent phenomenon of selforganization that is exclusively driven by optical thermalization toward the RayleighJeans equilibrium.



Braun, S., Asenjo, F. A., & Mahajan, S. M. (2014). Comment on “SpinGradientDriven Light Amplification in a Quantum Plasma” Reply. Phys. Rev. Lett., 112(12), 1 pp.



Comisso, L., & Asenjo, F. A. (2014). ThermalInertial Effects on Magnetic Reconnection in Relativistic Pair Plasmas. Phys. Rev. Lett., 113(4), 5 pp.
Abstract: The magnetic reconnection process is studied in relativistic pair plasmas when the thermal and inertial properties of the magnetohydrodynamical fluid are included. We find that in both SweetParker and Petschek relativistic scenarios there is an increase of the reconnection rate owing to the thermalinertial effects, both satisfying causality. To characterize the new effects we define a thermalinertial number which is independent of the relativistic Lundquist number, implying that reconnection can be achieved even for vanishing resistivity as a result of only thermalinertial effects. The current model has fundamental importance for relativistic collisionless reconnection, as it constitutes the simplest way to get reconnection rates faster than those accessible with the sole resistivity.



Concha, A., Aguayo, D., & Mellado, P. (2018). Designing Hysteresis with Dipolar Chains. Phys. Rev. Lett., 120(15), 5 pp.
Abstract: Materials that have hysteretic response to an external field are essential in modern information storage and processing technologies. A myriad of magnetization curves of several natural and artificial materials have previously been measured and each has found a particular mechanism that accounts for it. However, a phenomenological model that captures all the hysteresis loops and at the same time provides a simple way to design the magnetic response of a material while remaining minimal is missing. Here, we propose and experimentally demonstrate an elementary method to engineer hysteresis loops in metamaterials built out of dipolar chains. We show that by tuning the interactions of the system and its geometry we can shape the hysteresis loop which allows for the design of the softness of a magnetic material at will. Additionally, this mechanism allows for the control of the number of loops aimed to realize multiplevalued logic technologies. Our findings pave the way for the rational design of hysteretical responses in a variety of physical systems such as dipolar cold atoms, ferroelectrics, or artificial magnetic lattices, among others.



Mason, P., Josserand, C., & Rica, S. (2012). Activated Nucleation of Vortices in a DipoleBlockaded Supersolid Condensate. Phys. Rev. Lett., 109(4), 5 pp.
Abstract: We investigate theoretically and numerically a model of a supersolid in a dipoleblockaded BoseEinstein condensate. The dependence of the superfluid fraction with an imposed thermal bath and a uniform boost velocity on the condensate is considered. Specifically, we observe a critical velocity for the nucleation of vortices in our system that is strongly linked to a steplike decrease in the superfluid fraction. We are able to use a scaling argument based on the energy required to activate a vortex, relating the critical temperature to the critical velocity, and find that this relationship is in good agreement with the numerical simulations carried out on the nonlocal GrossPitaevskii equation.



Mellado, P., Concha, A., & Rica, S. (2020). Magnetoelectric Effect in Dipolar Clusters. Phys. Rev. Lett., 125(23), 237602.
Abstract: We combine the anisotropy of magnetic interactions and the point symmetry of finite solids in the study of dipolar clusters as new basic units for multiferroics metamaterials. The Hamiltonian of magnetic dipoles with an easy axis at the vertices of polygons and polyhedra, maps exactly into a Hamiltonian with symmetric and antisymmetric exchange couplings. The last one gives rise to a DzyaloshinskiiMoriya contribution responsible for the magnetic modes of the systems and their symmetry groups, which coincide with those of a particle in a crystal field with spinorbit interaction. We find that the clusters carry spin current and that they manifest the magnetoelectric effect. We expect our results to pave the way for the rational design of magnetoelectric devices at room temperature

