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Register a new userMagnetic monopole and string excitations in a two-dimensional spin ice
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Winder Alexander Moura-Melo
Publication date
2009
fields
Physics
and research's language is
English
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We study the magnetic excitations of a square lattice spin-ice recently produced in an artificial form, as an array of nanoscale magnets. Our analysis, based upon the dipolar interaction between the nanomagnetic islands, correctly reproduces the ground-state observed experimentally. In addition, we find magnetic monopole-like excitations effectively interacting by means of the usual Coulombic plus a linear confining potential, the latter being related to a string-like excitation binding the monopoles pairs, what indicates that the fractionalization of magnetic dipoles may not be so easy in two dimensions. These findings contrast this material with the three-dimensional analogue, where such monopoles experience only the Coulombic interaction. We discuss, however, two entropic effects that affect the monopole interactions: firstly, the string configurational entropy may loose the string tension and then, free magnetic monopoles should also be found in lower dimensional spin ices; secondly, in contrast to the string configurational entropy, an entropically driven Coulomb force, which increases with temperature, has the opposite effect of confining the magnetic defects.
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Time-of-flight inelastic neutron scattering measurements on Sr2IrO4 single crystals were performed to access the spin Hamiltonian in this canonical Jeff=1/2 spin-orbital Mott insulator. The momentum of magnetic scattering at all inelastic energies that were measured is revealed to be $L$-independent, indicative of idealized two-dimensional in-plane correlations. By probing the in-plane energy and momentum dependence up to ~80 meV we model the magnetic excitations and define a spin-gap of 0.6(1) meV. Collectively the results indicate that despite the strong spin-orbit entangled isospins an isotropic two-dimensional S=1/2 Heisenberg model Hamiltonian accurately describes the magnetic interactions, pointing to a robust analogy with unconventional superconducting cuprates.
Magnetic monopoles have eluded experimental detection since their prediction nearly a century ago by Dirac. Recently it has been shown that classical analogues of these enigmatic particles occur as excitations out of the topological ground state of a model magnetic system, dipolar spin ice. These quasi-particle excitations do not require a modification of Maxwells equations, but they do interact via Coulombs law and are of magnetic origin. In this paper we present an experimentally measurable signature of monopole dynamics and show that magnetic relaxation measurements in the spin ice material $Dy_{2}Ti_{2}O_{7}$ can be interpreted entirely in terms of the diffusive motion of monopoles in the grand canonical ensemble, constrained by a network of Dirac strings filling the quasi-particle vacuum. In a magnetic field the topology of the network prevents charge flow in the steady state, but there is a monopole density gradient near the surface of an open system.
One of the most remarkable examples of emergent quasi-particles, is that of the fractionalization of magnetic dipoles in the low energy configurations of materials known as spin ice, into free and unconfined magnetic monopoles interacting via Coulombs 1/r law [Castelnovo et. al., Nature, 451, 42-45 (2008)]. Recent experiments have shown that a Coulomb gas of magnetic charges really does exist at low temperature in these materials and this discovery provides a new perspective on otherwise largely inaccessible phenomenology. In this paper, after a review of the different spin ice models, we present detailed results describing the diffusive dynamics of monopole particles starting both from the dipolar spin ice model and directly from a Coulomb gas within the grand canonical ensemble. The diffusive quasi-particle dynamics of real spin ice materials within quantum tunneling regime is modeled with Metropolis dynamics, with the particles constrained to move along an underlying network of oriented paths, which are classical analogues of the Dirac strings connecting pairs of Dirac monopoles.
Artificial spin ices (ASIs) are interacting arrays of lithographically-defined nanomagnets in which novel frustrated magnetic phases can be intentionally designed. A key emergent description of fundamental excitations in ASIs is that of magnetic monopoles -- mobile quasiparticles that carry an effective magnetic charge. Here we demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, high-density plasma-like regimes of mobile magnetic monopoles. By passively listening to spontaneous monopole noise in thermal equilibrium, we reveal their intrinsic dynamics and show that monopole kinetics are minimally correlated (that is, most diffusive) in the plasma phase. These results open the door to on-demand monopole regimes having field-tunable densities and dynamic properties, thereby providing a new paradigm for probing the physics of effective magnetic charges in synthetic matter.
We have studied the spin dynamics of a high-mobility two-dimensional electron system in a GaAs/Al_{0.3}Ga_{0.7}As single quantum well by time-resolved Faraday rotation and time-resolved Kerr rotation in dependence on the initial degree of spin polarization, P, of the electrons. By increasing the initial spin polarization from the low-P regime to a significant P of several percent, we find that the spin dephasing time, $T_2^ast$, increases from about 20 ps to 200 ps; Moreover, $T_2^ast$ increases with temperature at small spin polarization but decreases with temperature at large spin polarization. All these features are in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B {bf 68}, 075312 (2003)]. Measurements as a function of spin polarization at fixed electron density are performed to further confirm the theory. A fully microscopic calculation is performed by setting up and numerically solving the kinetic spin Bloch equations, including the Dyakonov-Perel and the Bir-Aronov-Pikus mechanisms, with {em all} the scattering explicitly included. We reproduce all principal features of the experiments, i.e., a dramatic decrease of spin dephasing with increasing $P$ and the temperature dependences at different spin polarizations.
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