arXiv subjects

Ricardo Zarzuela

Publications and source records attributed to Ricardo Zarzuela.

23 records · Page 2Linked to original sources

arXiv2016

A Realization of the Haldane-Kane-Mele Model in a System of Localized Spins

We theoretically study a spin Hamiltonian for spin-orbit-coupled ferromagnets on the honeycomb lattice. We find that the effective Hamiltonian for magnons, a quanta of spin-wave excitations from ordered states, is equivalent to the Haldane model for electrons in the honeycomb lattice, which indicates the nontrivial topology of the magnon bands and the existence of the associated edge state. We also take the Schwinger boson (or bosonic spinon) representations of spins that are applicable both to ordered and disordered phases. We show that the spinon bands within the mean-field treatment are topologically nontrivial, which causes a spin Nernst effect, by adopting the properties of the Kane-Mele model. A feasible experimental realization of the spin Hamiltonian is proposed.

cond-mat.mes-hall↗

arXiv2013

Quantum Forces in Molecular Magnets

Two-state systems may exhibit mechanical forces of purely quantum origin that have no counterpart in classical physics. We show that the such forces must exist in molecular magnets due to quantum tunneling between classically degenerate magnetic states. They can be observed in the presence of a microwave field when the magnet is placed in a static magnetic field with a gradient.

cond-mat.mes-hall↗

arXiv2012

Excitation modes of vortices in sub-micron magnetic disks

Classical and quantum theory of spin waves in the vortex state of a mesoscopic sub-micron magnetic disk has been developed with account of the finite mass density of the vortex. Oscillations of the vortex core resemble oscillations of a charged string in a potential well in the presence of the magnetic field. Conventional gyroscopic frequency appears as a gap in the spectrum of spin waves of the vortex. The mass of the vortex has been computed that agrees with experimental findings. Finite vortex mass generates a high-frequency branch of spin waves. Effects of the external magnetic field and dissipation have been addressed.

cond-mat.mes-hall↗

arXiv2011

Quantum depinning of the magnetic vortex core in micron-size permalloy disks

The vortex state, characterized by an in-plane closed flux domain structure and an out-of-plane magnetization at its centre (the vortex core), is one of the magnetic equilibria of thin soft ferromagnetic micron-size dots. In the last two decades many groups have been working on the dynamics of the magnetic moment in nanomagnetic materials at low temperatures, it giving rise to the observation of quantum relaxations and quantum hysteresis cycles. For the first time, we report experimental evidence of quantum dynamics of the vortex core of micron-size permalloy (Fe$_{19}$Ni$_{81}$) disks induced by the application of an in-plane magnetic field. It is attributed to the quantum tunneling of the vortex core through pinning barriers, which are associated to structural defects in the dots, towards its equilibrium position.

cond-mat.mes-hall↗

arXiv2011

Magnetic field dependence of the quantum tunneling of normal-superconductor interfaces in a type-I Pb superconductor

We report experimental evidence of the effect of an applied magnetic field on the non-thermal magnetic relaxation in a disk-shaped type-I lead superconductor. The time evolution of the irreversible magnetization proves to be logarithmic for a wide range of temperatures and magnetic field values along the descending branch of the hysteresis cycle. When the intensity of the magnetic field increases, the crossover temperature separating the thermal and non-thermal regimes of magnetic relaxation is found to decrease, whereas the rate at which such relaxation occurs is observed to increase. These results are discussed in the framework of a recent model for quantum tunneling of normal-superconductor interfaces through the distribution of pinning energy barriers generated by structural defects in the sample, considering that the strength of the barriers decreases with the magnetic field. A phase diagram describing the dynamics of interfaces during flux expulsion in the intermediate state as a function of temperature and magnetic field is constructed.

cond-mat.supr-con↗