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arXiv:quant-ph/0110113 · Physical review. B, Condensed matter

ac-driven quantum spins: Resonant enhancement of transverse dc magnetization

Sergej Flach A, Andrey E. Miroshnichenko, Alexander A. Ovchinnikov

Published Mar 1, 2002• 8 citations
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Abstract

We consider $s=1/2$ spins in the presence of a constant magnetic field in the z direction and an ac magnetic field in the $x\ensuremath{-}z$ plane. A nonzero dc magnetization component in the y direction is a result of broken symmetries. A pairwise interaction between two spins is shown to resonantly increase the induced magnetization by one order of magnitude. We discuss the mechanism of this enhancement, which is due to additional avoided crossings in the level structure of the system.

DOI
10.1103/physrevb.65.104438

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We consider s=1/2s=1/2s=1/2 spins in the presence of a constant magnetic field in the z direction and an ac magnetic field in the x\ensuremath−zx\ensuremath{-}zx\ensuremath−z plane. A nonzero dc magnetization component in the y direction is a result of broken symmetries. A pairwise interaction between two spins is shown to resonantly increase the induced magnetization by one order of magnitude. We discuss the mechanism of this enhancement, which is due to additional avoided crossings in the level structure of the system.
We investigate second-order magnetic responses of quantum magnets against ac magnetic fields. We focus on the case where the zzz component of the spin is conserved in the unperturbed Hamiltonian and the driving field is applied in the xyxyxy plane. We find that linearly polarized driving fields induce a second-harmonic response, while circularly polarized fields generate only a zero-frequency response, leading to a magnetization with a direction determined by the helicity. Employing an unbiased numerical method, we demonstrate the nonlinear magnetic effect driven by the circularly polarized field in the XXZ model and show that the magnitude of the magnetization can be predicted by the dynamical spin structure factor in the linear response regime.
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We investigate the effect of a unidirectional quenched random field on the anisotropic quantum spin-1/2 XYXYXY model, which magnetizes spontaneously in the absence of the random field. We adopt a mean-field approach for this analysis. In general, the models considered have Ising symmetry, and as such they exhibit ferromagnetic order in two and three dimensions in the presence of not too large disorder. Even in the special case when the model without disorder has U(1)\text{U}(1)U(1) symmetry, a small U(1)\text{U}(1)U(1)-symmetry-breaking random field induces ferromagnetic long-range order in two dimensions. The mean-field approach, consequently, provides a rather good qualitative and even quantitative description when applied not too close to the criticality. We show that spontaneous magnetization persists even in the presence of the random field, but the magnitude of magnetization gets suppressed due to disorder, and the system magnetizes in the directions parallel and transverse to the random field. Our results are obtained via analytical calculations within a perturbative framework and by numerical simulations. Interestingly, we show that it is possible to enhance a component of magnetization in the presence of the disorder field provided that we apply an additional constant field in the XYXYXY plane. Moreover, we derive generalized expressions for the critical temperature and the scalings of the magnetization near the critical point for the XYXYXY spin system with arbitrary fixed quantum spin angular momentum.
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